The PEARL study: a prospective two-group pilot PrEP promotion intervention for cisgender female sex workers living in Baltimore, MD, U.S.

HIV remains elevated among female sex workers (FSW) globally, with a number of structural (e.g., poverty, access to care) factors driving these persistently high rates. Pre-exposure prophylaxis (PrEP), a user-controlled prevention method, is a promising means of empowering vulnerable populations to protect themselves and enhance agency. Yet there is a dearth of PrEP research and interventions targeting cisgender women in the United States, and even fewer aimed to reach FSW. We developed and implemented a multifaceted PrEP pilot intervention, the Promoting Empowerment And Risk Reduction (PEARL) study, to meet this gap. This paper describes the development process and nature of a community-informed intervention for tenofovir/emticitrabine (TDF/FTC) pre-exposure prophylaxis engagement among street-based cisgender FSW in Baltimore, Maryland, U.S. In the course of the study's implementation, structural, programmatic, and medical barriers have already posed significant barriers to full engagement. PEARL implemented a number of strategies in an effort to counter barriers and facilitate increased success of PrEP uptake and maintenance. The study will provide critical insights into the nature of intervention components that could help FSW to initiate PrEP and reduce PrEP care cascade gaps.

Risk factors associated with resistance to HIV testing among transwomen in Brazil.

Transwomen are a high-risk population for HIV/AIDS worldwide. However, many transwomen do not test for HIV. This study aimed to identify factors associated with resistance to HIV testing among transwomen in Fortaleza/CE. A cross-sectional study was conducted between August and December 2008 with a sample of 304 transwomen recruited through respondent-driven sampling. Data analysis utilized Respondent-Driven Sampling Analysis Tool and SPSS 11.0. Univariate, bivariate, and multivariate analyses examined risk factors associated with resistance to HIV testing. Less than 18 years of age (OR = 4.221; CI = 2.419-7.364), sexual debut before 10 years of age (OR = 6.760; CI = 2.996-15.256), using illegal drugs during sex (OR = 2.384; CI = 1.310-4.339), experience of discrimination (OR = 3.962; CI = 1.540-10.195) and a belief that the test results were not confidential (OR = 3.763; CI = 2.118-6.688) are independently associated with resistance to testing. Intersectoral and targeted strategies aimed at encouraging the adoption of safer sexual behaviors and testing for HIV among transwomen are required.

Linguistic structure and transposition.

Transfer of a learned discrimination along the size dimension was studied in groups of American and African tribal children. The language spoken by the African children contains an asymmetry in the expression of size comparisons that is not present in English. Contrary to theories of linguistic mediation of choice behavior, transfer choices were not related to the differing linguistic patterns; however, initial choices and post-test descriptions of choices were so related.

Conjunctivitis outbreak among divers.

In March 2006, an outbreak of conjunctivitis that occurred over a six day period among twenty-nine individuals who partook in recreational scuba diving trips on two boats off Vitu Levu Island, Fiji. We investigated the likelihood that a communal container used to store diving masks facilitated the spread of conjunctivitis among individuals. The diagnosis of conjunctivitis was based on clinical assessment by a physician. Transmission of conjunctivitis from person to person was documented with eventual identification of the index case, the dive master, a Fijian resident. Topical antibiotics were dispensed accordingly and detergent and bleach were used as mask cleaning agents in an effort to control the outbreak. Follow up surveys were mailed to all twenty-nine participants. Ultimately, fourteen cases of conjunctivitis were documented (46.7%). Eleven cases were verified during the six days in Fiji, two upon arrival back in the U.S., and one case of familial transmission in the U.S. All but two cases resolved within one week. Unknown to these divers was a coincidental, generalized outbreak of acute haemorrhagic conjunctivitis among the Fijian Residents. The communal container used to store diving masks was the likely vector for the spread of infectious conjunctivitis, the first such documented outbreak involving communal diving equipment.

Liver-directed gene transfer and prolonged expression of three major human ApoE isoforms in ApoE-deficient mice.

Apolipoprotein E (apoE) plays a key role in lipoprotein metabolism and may have other important biological functions. In humans, there are three common, naturally occurring isoforms of apoE that are associated with differences in lipid levels and atherosclerosis. However, the direct in vivo effects of the apoE isoforms on lipoprotein metabolism and atherosclerosis are not yet fully understood. To investigate the effect of the apoE isoforms in vivo, we constructed second-generation recombinant adenoviruses encoding each of the apoE isoforms. These recombinant adenoviruses were injected intravenously into apoE-deficient mice fed a Western diet (mean baseline cholesterol level 1401 mg/dl) in order to study their effects in the absence of endogenous mouse apoE. Hepatic expression of apoE3 and apoE4 completely normalized the lipoprotein profile; 3 d after injection, mean plasma cholesterol levels were 194 and 217 mg/ dl, respectively, and this effect was maintained for at least 6 wk. Expression of apoE2 had much less effect on lipoprotein levels (mean cholesterol level 752 mg/dl 3 d after injection), despite much higher plasma levels of apoE2 compared with apoE3 and apoE4; by 6 wk after injection the cholesterol levels had returned to baseline levels in the apoE2-expressing mice. Expression of all three isoforms significantly increased HDL cholesterol levels by approximately threefold and was independent of the cholesterol-lowering effect. ApoE transgene expression was substantially prolonged compared with that achieved using a first generation adenovirus and apoE was readily detected in plasma 3 mo after virus injection. These studies demonstrate: (a) prolonged in vivo expression of human apoE isoforms in apoE deficient mice after second-generation recombinant adenovirus-mediated somatic gene transfer; and (b) significantly impaired ability of apoE2 in vivo to mediate clearance of remnant lipoproteins in apoE-deficient mice fed a Western diet compared with apoE3 and apoE4.

Phase II study of taxol, merbarone, and piroxantrone in stage IV non-small-cell lung cancer: The Eastern Cooperative Oncology Group Results.

BACKGROUND: Patients with metastatic (stage IV) non-small-cell lung cancer usually have a poor prognosis and disease refractory to chemotherapy. Three new agents--taxol, merbarone, and piroxantrone--have shown promising antitumor treatment in vitro and in animals. Taxol is an antimicrotubular agent that interferes with mitosis during cell division. Merbarone, a conjugate of thiobarbituric acid and aniline, is a topoisomerase II inhibitor, which thus inhibits DNA synthesis and tumor growth. Piroxantrone, an anthracenedione derivative, is a DNA intercalating agent that has shown potent antitumor activity in animal studies. PURPOSE: Our randomized phase II study was designed to evaluate the efficacy and toxicity of these agents in the treatment of stage IV metastatic non-small-cell lung cancer. METHODS: Eligible patients (119) were randomly assigned to receive one of the three treatments given every 3 weeks: 250 mg/m2 taxol by a 24-hour intravenous infusion, 1000 mg/m2 merbarone by continuous intravenous infusion through a central catheter daily for 5 days, or 150 mg/m2 piroxantrone by intravenous infusion over 1 hour. Patients had received no chemotherapy. Response and toxicity were evaluated every 3 weeks. RESULTS: Twenty-five patients were randomly assigned to receive taxol, 47 to receive merbarone, and 47 to receive piroxantrone. One of 44 assessable patients (2.3%) treated with piroxantrone had a complete response. Rates for partial response were 20.8% (five of 24 patients) and 5.7% (two of 35) for assessable patients treated with taxol or merbarone, respectively. One-year survival rates were 41.7%, 21.6%, and 22.6%, and median survival times were 24.1, 19.9, and 29.3 weeks for taxol, merbarone, and piroxantrone, respectively. These differences were not statistically significant, but this study was not designed to compare survival. In general, toxicity was manageable. With premedication, no anaphylaxis was observed with taxol. The most common toxic effects were leukopenia with taxol or piroxantrone treatment and thromboembolic complications with merbarone. Death directly related to treatment occurred in 4% (one patient), 11.4% (four), and 5% (two) of the assessable patients receiving taxol, merbarone, and piroxantrone, respectively. Cardiotoxicity and neurotoxicity occurred only occasionally in all three arms. CONCLUSION: On the basis of the response rate (20.8% partial response) and 1-year survival rate (41.7%), taxol is an active agent for the treatment of metastatic non-small-cell lung cancer. Merbarone and piroxantrone are relatively inactive. IMPLICATIONS: Further study of taxol is warranted. In future studies, taxol should be combined with other agents, and granulocyte colony-stimulating factor should be used to ameliorate myelosuppression.

A multipronged, nutritional-based strategy for managing Alzheimer's disease.

A nutritional-based strategy has been proposed in order to improve cognitive performance of Alzheimer's disease (AD) patients. The strategy requires daily dietary supplementation with magnesium (Mg), folic acid, and vitamins B6 and B12, daily consumption of silicic acid-rich mineral water in order to lower the body burden of Al, and several plasma exchange procedures in order to replace Aß-bound albumin with fresh albumin. Evidence suggests that the deteriorating cognitive performance associated with AD may be improved by supplementation with either Mg alone or with the combination of the above three B vitamins (B vitamin combo), or by drinking silicic acid-rich mineral water, or by undergoing plasma exchange. However, for the following reasons the combination of all four therapeutic approaches may have a synergistic effect on improving cognitive performance of AD patients.

tRNA methyltransferases from rat liver. Differences in response of partially purified enzymes to polyamines and inorganic salts.

Three tRNA methyltransferases, purified from rat liver, have been compared for their activity in the presence of various amines and Mg2+. The enzymes differ with respect to the ion which permits maximal activity; they also differ with respect to the concentration of a given ion necessary for maximal activity. The methyltransferase which forms N2-methylguanine in the region between the dihydrouridine loop and the acceptor stem (2mG I), when assayed using purified tRNA as substrate, shows high activity with 3--5 mM sperimidine or 20 mM putrescine and significantly lower rates of methylation with 200--350 mM ammonium acetate or 1--10 mM magnesium acetate. The enzyme responsible for forming N2-methylguanine between the dihydrouridine and anticodon loops (2mG II) works well in the presence of 0.2--0.5 mM spermidine, 10 mM putrescine or 200--300 mM ammonium acetate and shows slightly lower activity with 1 mM magnesium acetate. The optimal conditions for assaying 1-adenine methyltransferase (1mA) with purified tRNAs are either 200--300 mM ammonium acetate or 30 mM putrescine; spermidine is slightly less effective and magnesium acetate permits less than 25% of maximal activity. The addition of 10 mM Mg2+, in combination with polyamines or NH4+, depresses slightly the activity of the guanine methyltransferases but completely abolishes the polyamine or ammonium-stimulated activity of the adenine methyltransferase. When unfractionated (Escherichia coli) tRNA is used as substrate, the concentrations of polyamines required for optimal methyltransferase activity are increased but the patterns of response of the three enzymes do not differ significantly from those obtained with purified tRNA substrates. Based on the studies with these three enzymes, unfractionated tRNA and 40 mM putrescine should provide the most reliable system for detecting methylating activity if the nature of the tRNA methyltransferase is unknown.

Effects of metabolic inhibitors on the synthesis and release of lipoprotein lipase in cultured cells derived from the stromal-vascular fraction of rat adipose tissue.

Cultured cells derived from the stromal-vascular fraction of rat epididymal adipose tissue have been used to study the release of lipoprotein lipase in response to heparin. In the presence of heparin, lipoprotein lipase appeared in the medium biphasically, with an initial rapid rise followed by a slower linear release. The initial increase in activity in the medium was paralleled by a decrease in cell-associated activity. Over the course of the slower phase of release, cell-associated lipoprotein lipase levels remained constant. Treatment of cultures with cycloheximide or 2-deoxyglucose eliminated the second phase of release as well as cell-associated lipoprotein lipase activity. Treatment of cultures with Colcemid decreased the second phase of release but increased cell-associated lipoprotein lipase activity. These data suggest that exposure to heparin results in a rapid release of pre-formed enzyme and that continued release is dependent on both protein synthesis and transport of the enzyme to the cell surface.

Efflux of phospholipid from fibroblasts with normal and elevated levels of cholesterol.

To address the hypothesis that phospholipid efflux from cells contributes to lipoprotein structure, we have examined the efflux of biosynthetically labeled [32P]phospholipid from cells to lipoproteins. With normal human skin fibroblasts in monolayer culture, high density lipoprotein (HDL3) promoted the efflux of phospholipid in a concentration-dependent manner. As analyzed by TLC, the major phospholipids released from fibroblasts were phosphatidylcholine, sphingomyelin, lysophosphatidylcholine and phosphatidylethanolamine. At identical concentrations, HDL3 and dimethylsuberimidate treated-HDL3 promoted similar efflux, suggesting that efflux did not depend on specific binding of HDL3 to the cell surface. When the content of cholesterol in fibroblasts was doubled by pre-incubation with LDL and cholesterol-rich liposomes, the fractional efflux of phospholipid to HDL3 and other acceptors was stimulated about 2-fold. Most of this stimulation was due to enhanced release of phosphatidylcholine. Similar effects of enrichment were found with Fu5AH rat hepatoma cells, but not with J774 mouse macrophages. The results support the hypothesis that the efflux of phospholipid from cells contributes to the phospholipid content of HDL. This may enhance the ability of HDL to remove cholesterol from cells, the initial step in reverse cholesterol transport.

Effect of fatty acid supplementation on cholesterol and retinol esterification in J774 macrophages.

J774 macrophages exposed to medium containing cholesterol-rich phospholipid dispersions accumulate cholesteryl ester. Supplementing this medium with 100 micrograms oleate/ml increased cellular cholesteryl ester contents 3-fold. Cell retinyl ester contents increased 8-fold when medium containing retinol dispersed in dimethyl sulfoxide was supplemented with oleate. These increases were not the result of increases in total lipid uptake by the cells but rather of redistribution of cholesterol and retinol into their respective ester pools. Effective oleate concentration of 15-30 micrograms/ml increased cellular retinyl and cholesteryl ester contents. The effective oleate concentration was reduced to 5 micrograms/ml when the fatty acid/albumin molar ratio was increased. The oleate-stimulated increase in cholesterol esterification was blocked by incubating cells with Sandoz 58-035, a specific inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), indicating that the effect of fatty acid exposure is mediated through changes in ACAT activity. When cholesterol or retinol was added to cells which had been exposed to oleate for 24 h to provide a triacylglycerol store, the cellular contents of cholesteryl or retinyl ester were also significantly increased compared to cells not previously exposed to oleate. The oleate-stimulated increase in the esterification of cholesterol and/or retinol was also observed in P388D1 macrophages, human (HepG2) and rat (Fu5AH) hepatomas, human fibroblasts, rabbit aortic smooth muscle cells and MCF-7 breast carcinoma cells. In addition to oleate, a number of other fatty acids increased retinol esterification in J774 macrophages; however, cellular cholesterol esterification in these cells was increased only by unsaturated fatty acids and was inhibited in the presence of saturated fatty acids. Although the cellular uptake of radiolabeled oleate and palmitate was similar, a significant difference in the distribution of these fatty acids among the lipid classes was observed. These data demonstrate that exogenous fatty acids are one factor that regulate cellular cholesteryl and retinyl ester contents in cultured cells.

Hydrolysis of lipid monolayers and the substrate specificity of hepatic lipase.

The substrate specificities of the phospholipase and triglyceridase activities of purified rat liver hepatic lipase were compared using lipid monolayers so that the substrates were presented to the enzyme in a controlled physical state. The rate of hydrolysis of 14C-labeled lipid at constant surface pressure in the presence of hepatic lipase and fatty acid-free bovine serum albumin at 33 degrees C was determined by monitoring the decrease of surface radioactivity. In monolayers of sphingomyelin/cholesterol (2:1, mol/mol) containing either 1 mol% triacylglycerol, 1 mol% phosphatidylethanolamine, or 10 and 20 mol% phosphatidylcholine, hepatic lipase clearly showed a preference for unsaturated over saturated lipids. In addition, with a sphingomyelin/cholesterol (2:1) monolayer containing 1 mol% of lipid substrate, hepatic lipase showed the following preference: triolein = dioleoylphosphatidylethanolamine much greater than dioleoylphosphatidylcholine; the respective rates of hydrolysis were 15.3 +/- 1.2, 14.9 +/- 0.8, and 0.5 +/- 0.1 mumol fatty acid produced/h per mg hepatic lipase. Overall, it appears that when comparing rates of hydrolysis of molecules within a given lipid class, hydrocarbon chain interactions are important. However, when comparing different lipid classes such as phosphatidylcholines and phosphatidylethanolamines, it is apparent that the polar group has a significant influence on the rate of hydrolysis. The rate of [14C]triolein hydrolysis, when mixed at surface concentrations of up to 2 mol% in a sphingomyelin/cholesterol (2:1) monolayer, was significantly faster than when triolein was present in a 1-oleyl-2-palmitylphosphatidylcholine monolayer; the rates of hydrolysis were 47.7 +/- 5.4 and 8.9 +/- 0.8 mumol fatty acid produced/h per mg hepatic lipase, respectively. The monolayer physical state and the miscibility of the substrate in the inert matrix influence the presentation of the substrate to the enzyme, thereby affecting the hydrolysis rate.

Incubation of acetylated low-density lipoprotein with cholesterol-rich dispersions enhances cholesterol uptake by macrophages.

Incubation of J774 macrophages with mixtures of acetylated low-density lipoprotein (acLDL) and free cholesterol-rich phospholipid dispersions increases cellular cholesterol deposition 2-4-fold over that achieved with either acLDL or dispersions alone. Both free and esterified cholesterol accumulate in cells incubated with the mixture of acLDL and dispersions. A similar result is observed when acLDL is replaced by malondialdehyde-LDL. The enhanced deposition of cholesterol is not unique to J774 macrophages, as P388D1 macrophages also accumulate more cholesterol when incubated with the mixture of acLDL and dispersions than either particle alone. A preincubation of the particles for at least 6 h prior to incubation with cells is required in order to observe maximal cholesterol delivery. Both dispersion free cholesterol and phospholipid accumulate in J774 cells, suggesting that a complex is formed between acLDL and dispersions which results in a cholesterol-rich acLDL/dispersion particle. Partial purification of the acLDL-dispersion complex revealed increases in the size distribution of the particles compared to acLDL and increases in free cholesterol and phospholipid contents. Cholesterol uptake from the mixture of acLDL and dispersions was saturable and the enhanced cellular uptake of both cholesterol and phospholipid from the complex could be abolished by inhibitors of the scavenger receptor pathway. In addition to the receptor-mediated uptake of cholesterol from the acLDL-dispersion complex, it was observed that approx. 30% of the total cholesterol uptake from the complex was via non-specific components, including surface transfer.

Metabolism of cholesteryl ester lipid droplets in a J774 macrophage foam cell model.

J774 macrophages rapidly incorporated [3H]cholesteryl oleate droplets by a non-saturable phagocytic process. In less than 2 h, foam cell morphology was acquired. The extent of loading obtained after 2 h was a linear function of the mass of cholesteryl oleate provided to the cells. The cholesteryl oleate incorporated was hydrolyzed in the cells at a linear rate over 24 h and the fractional hydrolysis was constant over a wide range of cellular esterified cholesterol contents. The rate of hydrolysis was influenced by the physical state of the cholesteryl ester; cholesteryl oleate in isotropic droplets was hydrolyzed 2-3-fold more rapidly than cholesteryl oleate in anisotropic droplets. The hydrolysis of both types of droplets was inhibited by lysosomotropic agents, indicating that hydrolysis occurred in the lysosomes. Only a small fraction (less than 10% after 24 h) of the free [3H]cholesterol generated in the lysosomes was esterified by ACAT resulting in a doubling of the cell free cholesterol content. Electron microscopy of cells treated with digitonin revealed the accumulation of free cholesterol in lipid-laden lysosomes. ACAT was active as endogenous free [14C]cholesterol was esterified in a linear manner over 24 h and was responsive to the presence of lysosomally-derived cholesterol, as the extent of esterification of the endogenous pool was directly proportional to the mass of [3H]cholesterol generated in the lysosomes.

Purification and characterization of two tRNA-(guanine)-methyltransferases from rat liver.

tRNA(guanine-1-)-methyltransferase (EC 2.1.1.31) and tRNA(N2-guanine)-methyltransferase I (EC 2.1.1.32) were isolated from rat liver. The (guanine-1-)-methyltransferase preparation is 6800-fold purified and is free from contaminating methyltransferases or ribonuclease. The molecular weight of (guanine-1-)-methyltransferase is 83 000. Of seven purified Escherichia coli tRNAs examined, only tRNAMetf was utilized as substrate by (guanine-1-)-methyltransferase. The methylation of tRNAMetf is maximally stimulated by 40 mM putrescine with a pH optimum of 8.0. Using E. coli K-12 tRNA, the Km for S-adenosylmethionine is 3 micrometer and Ki for S-adenosylhomocysteine is 0.11 micrometer for (guanine-1-)-methyltransferase. (N2-Guanine-)-methyltransferase is 6200-fold purified and is also free of interfering enzymes. It has a molecular weight of 69 000. E. coli tRNAPhe, tRNAVal and tRNAArg are substrates for this enzyme which introduces a methyl at the 2-amino group of the guanine at position 10 from the 5'-terminus of these tRNAs. The methylation of tRNAPhe is maximally stimulated by 100 micrometer spermidine with a pH optimum of 8.0. (N2-Guanine-)-methyltransferase has a Km for S-adenosylmethionine of 2 micrometer and a Ki for S-adenosylhomocysteine of 23 micrometer with E. coli K-12 tRNA as methyl acceptor.

Effect of substrate physical state on the activity of acid cholesteryl ester hydrolase.

The effects of the physicochemical properties of the substrate vehicle on the activity of acid cholesteryl ester hydrolase (ACEH; EC 3.1.1.13) isolated from rat liver lysosomes have been studied. In particular, the influence of the physical state of the neutral lipid core of substrate emulsion particles on the enzymatic activity has been probed in the light of previous studies on the clearance of cholesteryl esters (CE) from lipid-loaded cells which indicated that inclusions that are in the isotropic (liquid) state can be hydrolyzed faster than those in the anisotropic (liquid-crystalline) state. In the present study, such lipid inclusions were isolated from cultured cells and used as substrates for the hydrolase. No appreciable difference between the hydrolysis rates of isotropic and anisotropic inclusions was observed; the Vmax values were 93.0 +/- 6.7 and 84.0 +/- 3.3 nmol CE/mg.h, respectively. To elucidate the factors which affect the activity of ACEH, model inclusions were prepared by sonication and used as substrates. The physical state of these models was varied in a systematic way by changes of droplet composition and incubation temperature. The rate of hydrolysis was found to be insensitive to the physical state of the core of the model inclusions in good agreement with the results obtained with cellular inclusions. However, the activity of ACEH is sensitive to such interfacial properties of the lipid droplets as surface area available to the enzyme, net surface charge and surface solubility of the substrate CE molecules. The enzymatic activity is also sensitive to the amount of free cholesterol present in the emulsion droplets. The interfacial concentration and molecular packing of substrate CE molecules in the droplet surface significantly affect the hydrolytic activity of ACEH.

Characterization of a bile salt-dependent cholesteryl ester hydrolase activity secreted from HepG2 cells.

HepG2 cells and medium were assayed for cholesteryl ester hydrolase (CEH) activity in the presence and absence of sodium cholate. Although bile salt-dependent CEH activity was measured in the medium at 6 to 96 h (up to 4500 pmol/h per mg cell protein), there was very little activity detected in the corresponding cell homogenates (less than 70 pmol/h per mg cell protein). Activity in the medium was expressed only in the presence of trihydroxy bile salts and was maximal at 40 mM cholate and pH 7.5. Incubation of HepG2 cells with brefeldin A resulted in an 80 to 90% inhibition of secretion of the bile salt-dependent CEH activity, while only inhibiting total protein secretion by 42%. Bile salt-dependent CEH activity could also be detected in rat liver perfusates. Although there was measurable activity in all of 14 livers analyzed (47 +/- 10 and 53 +/- 17 nmol/h per g liver per h perfusion during two 5-min collections after 15 and 30 min of perfusion, respectively), it did not correlate with the activity found in corresponding liver homogenates, as only four livers had detectable bile salt-dependent CEH activity. These results provide evidence for the secretion of a bile salt-dependent CEH activity, from both a hepatic cell line and the intact liver, that has similar properties to the enzyme previously isolated from rat liver homogenates and rat pancreas.

S-adenosylhomocysteine analogues as inhibitors of specific tRNA methylation.

Of 17 base- or amino acid-modified analogues of S-adenosylhomocysteine, six were found to produce at least 50% inhibition of the activity of an unfractionated tRNA methyltransferase extract at concentrations of 200 micron. The inhibitory effects of these six analogues on five purified rat liver tRNA methyltransferases were examined. The purified enzymes differed greatly in their sensitivity to the analogues. Ki values for the inhibitory analogues were determined for the three most highly purified methyltransferases. The kinetic analyses indicated that inhibition is competitive for nearly all enzyme/inhibitor combinations. The Ki values for good enzyme/inhibitor pairs were in the range of 0.11--2 micron. Each analogue appears to inhibit one methylation more strongly than others; e.g. the Ki values obtained for N6-methyl-S-adenosyl-L-homocysteine are approx. 0.4 micron for guanine-1 tRNA methyltransferase, 6 micron for adenine-1 tRNA methyltransferase and 100 micron for N2-guanine tRNA methyltransferase I. Structural features which are important for inhibitory activity are presence of a terminal amino group on the amino acid and the presence of adenosine rather than any other base. Ring nitrogens, a terminal carboxyl group and conformation at the asymmetric carbon appear to be important for some but not all of the tRNA methyltransferases examined.