The Cape Triage Score -- a triage system for South Africa.

The Cape Triage Score (CTS) has been derived by the Cape Triage Group (CTG) for use in emergency units throughout South Africa. It can also be used in the pre-hospital setting, although it is not designed for mass casualty situations. The CTS comprises a physiologically based scoring system and a list of discriminators, designed to triage patients into one of five priority groups for medical attention. Three versions have been developed, for adults, children and infants.

Effects of a 100 metacercarial cyst inoculum on the host-parasite relationship of Echinostoma caproni and ICR mice.

The host-parasite relationship of a 100 metacercarial cyst inoculum of Echinostoma caproni in the ICR mouse was examined. Three groups of mice, A, B and C, each with six mice per group were used and all mice were necropsied at 14 days postinfection (p.i.), at which time the worms were ovigerous. Group A consisted of uninfected controls, whereas group B received 25 cysts per mouse (low dose) and group C received 100 cysts per mouse (high dose). There was no significant difference in food consumption between any of the groups from 0 to 14 days p.i. Control mice increased their body weight by 12%, group B by 5%, and group C showed a less than 1% increase in body weight between 0 and 14 days p.i. Echinostome parasitism caused a significant increase in the diameter of the mouse gut, with the gut of group C being more significantly dilated than that of either group A or B. The average worm recovery from group B was 20 worms per host, compared to 72 worms per host from group C. The mean wet and dry weights per worm from group B were 2.4 and 0.4 mg, respectively as compared to 0.6 and 0.2 mg respectively for group C. The mean number of uterine eggs per worm from group B was 180 compared to 125 for worms from group C. Worms from group C were more widely distributed in the small intestine than those from group B. Crowding effects associated with the high dose infection were clearly demonstrated in E. caproni from ICR mice.

Kinetics and equilibria in ligand binding by nitrophorins 1-4: evidence for stabilization of a nitric oxide-ferriheme complex through a ligand-induced conformational trap.

Nitrophorins 1-4 (NP1-4) are ferriheme proteins from the blood-sucking insect Rhodnius prolixus that transport nitric oxide (NO) to the victim, sequester histamine, and inhibit blood coagulation. Here, we report kinetic and thermodynamic analyses for ligand binding by all four proteins and their reduction potentials. All four undergo biphasic association and dissociation reactions with NO. The initial association is fast (1.5-33 microM(-)(1) s(-)(1)) and similar to that of elephant metmyoglobin. However, unlike in metmyoglobin, a slower second phase follows ( approximately 50 s(-)(1)), and the stabilized final complexes are resistant to autoreduction (E degrees = +3 to +154 mV vs normal hydrogen electrode). NO dissociation begins with a slow, pH-dependent step (0.02-1.4 s(-)(1)), followed by a faster phase that is again similar to that of metmyoglobin (3-52 s(-)(1)). The equilibrium dissociation constants are quite small (1-850 nM). NP1 and NP4 display larger release rate constants and smaller association rate constants than NP2 and NP3, leading to values for K(d) that are about 10-fold greater. The results are discussed in light of the recent crystal structures of NP1, NP2, and NP4, which display open, polar distal pockets, and of NP4-NO, which displays an NO-induced conformational change that leads to expulsion of solvent and complete burial of the NO ligand in a now nonpolar distal pocket. Taken together, the results suggest that tighter NO binding in the nitrophorins is due to the trapping of the molecule in a nonpolar distal pocket rather than through formation of particularly strong Fe-NO or hydrogen bonds.

Production of inhibin forms by the fetal membranes, decidua, placenta and fetus at parturition.

Inhibins are regulators of paracrine and endocrine function during pregnancy, but their intrauterine sites of secretion are not well established. In amniotic fluid, inhibin A-, inhibin B- and inhibin pro-alphaC-containing isoforms were present in high concentrations, whereas in maternal serum, inhibin A and pro-alphaC forms were present in high amounts, with low concentrations of inhibin B. In fetal cord serum, inhibin pro-alphaC was present in all samples, inhibin B was detectable in male but not female fetuses, with no detectable inhibin A in either sex. From cultured explants, both inhibin A and B were secreted by chorion laeve, whereas only inhibin A was secreted by placenta, with both tissues secreting inhibin pro-alphaC. Only low concentrations of both dimeric inhibins and pro-alphaC forms were secreted by decidua parietalis and amnion. The dual perfused placental cotyledon secreted both inhibin A and pro-alphaC into maternal perfusate, but only inhibin pro-alphaC into the fetal circulation and less than to the maternal side. We conclude that trophoblast is the predominant source of dimeric inhibins, but with markedly different secretion depending on its intrauterine location. There was a significant decrease in inhibin A and pro-alphaC in amniotic fluid collected at term active labour compared to elective Caesarean section (P < 0.001). This may reflect a local change in inhibin/activin processing at labour, likely in chorion laeve trophoblast cells, which may be important in the paracrine control of the feto-maternal communication required to maintain pregnancy and initiate labour.

Follistatin and activin A in extra-embryonic coelomic and amniotic fluids and maternal serum in early pregnancy.

Follistatin is a specific binding protein which controls bioavailability of activins and inhibins which have an important role in fetal development. In the first trimester of pregnancy bioactive dimeric inhibins are found at high concentrations in the extra-embryonic coelomic fluid, but the distribution of follistatin and activins is not known. We have used recently developed immunoassays for follistatin, activin A and activin AB to determine their presence in the intrauterine compartments during early pregnancy. Follistatin was present in highest concentrations in the extra-embryonic coelomic fluid (11.72 +/- 1.70 ng/ml; median +/- SEM), with less in maternal serum (6.35 +/- 4.58) and lowest amounts in amniotic fluid (0.97 +/- 0.52). Follistatin concentrations in extra-embryonic coelomic fluid were highly correlated with both dimeric inhibin isoforms. Activin A was present in only barely detectable amounts in some samples of extra-embryonic coelomic fluid (41% of samples) and maternal serum (26%) and was undetectable in all amniotic fluid samples. Activin AB was undetectable in all compartments. The presence of follistatin in the amniotic and extra-embryonic coelomic fluids may regulate the availability of bioactive activins and inhibins which are released into the intrauterine compartments during the development of the fetus and placenta in early pregnancy.

The crystal structure of nitrophorin 4 at 1.5 A resolution: transport of nitric oxide by a lipocalin-based heme protein.

BACKGROUND: Nitrophorins are nitric oxide (NO) transport proteins from the saliva of blood-feeding insects, which act as vasodilators and anti-platelet agents. Rhodnius prolixus, an insect that carries the trypanosome that causes Chagas' disease, releases four NO-loaded nitrophorins during blood feeding, whereupon the ligand is released into the bloodstream or surrounding tissue of the host. Histamine, a signaling molecule released by the host upon tissue damage, is tightly bound by the nitrophorins; this may facilitate the release of NO and reduce inflammation in the host. RESULTS: Recombinant nitrophorin 4 (NP4) was expressed in Escherichia coli, reconstituted with heme, and found to bind NO and histamine in a manner similar to that of the natural protein. The crystal structure of NP4 revealed a lipocalin-like eight-stranded beta barrel, with heme inserted into one end of the barrel. His59 ligates the proximal site on the heme, a solvent molecule (NH3) ligates the distal site, and three additional solvent molecules occupy the distal pocket. Buried in the protein interior are Glu55 and three solvent molecules. A detailed comparison with other lipocalins suggests that NP4 is closely related to the biliverdin-binding proteins from insects. CONCLUSIONS: The nitrophorins have a unique hemoprotein structure and are completely unlike the globins, the only other hemoproteins designed to transport dissolved gases. Compared with the recently described structure of NP1, the NP4 structure is considerably higher resolution, confirms the unusual placement of ionizable groups in the protein interior, and clarifies the solvent arrangement in the distal pocket. It also provides a striking example of structural homology where sequence homology is minimal.

Inhibin B in seminal plasma: testicular origin and relationship to spermatogenesis.

In men, inhibin B is the circulating isoform involved in the regulation of follicle stimulating hormone (FSH) secretion. Within the testis, inhibin B may have a role in Sertoli and germ cell interactions, thus secretion into seminal plasma may reflect seminiferous tubule function. Using specific immunoassays, inhibin B was present in seminal plasma in fertile men (n = 105) and in unselected men attending an infertility clinic (n = 174) with a wide range in concentration from undetectable (<15 pg/ml) up to 54,100 pg/ml (geometric mean 280 pg/ml). There was a highly significant correlation between seminal plasma inhibin B concentration and sperm concentration (r = 0.46, P < 0.001), but no correlation with percentages of spermatozoa with progressive motility or normal morphology. Inhibin A and isoforms containing pro and alphaC immunoreactivity were not detectable. In post-vasectomy seminal plasma samples (18 of 20) inhibin B was undetectable, indicating that the testis is the predominant source. In unselected men attending an infertility clinic, inhibin B was undetectable in 17% (present in remainder; maximum concentration 26,200 pg/ml; mean 263 pg/ml), with a highly significant correlation between seminal plasma inhibin B and sperm concentration (r = 0.55, P < 0.0001). In men with oligo/ azoospermia (sperm concentration <20 x 10(6)/ml), seminal plasma inhibin B concentrations were lower in those with elevated plasma FSH concentrations (mean values 42 and 205 pg/ml, P < 0.05). Inhibin alpha and betaB subunits were localized predominantly in Sertoli and Leydig cells, using immunohistochemistry. We conclude that inhibin B of testicular origin is present in normal human seminal plasma, but with a very wide range in concentration, and may reflect the functional state of the seminiferous epithelium.

Inhibin-B and pro-alphaC-containing inhibins in amniotic fluid from chromosomally normal and Down syndrome pregnancies.

In second-trimester Down syndrome pregnancies, levels of inhibin-A (the alpha-betaA dimer) in maternal serum and amniotic fluid (AF) are significantly higher and lower than in normal pregnancy, respectively. Since AF also contains inhibin-B (the alpha-betaB dimer) and precursor inhibins, we have examined whether the secretion of these inhibin isoforms may also be altered in association with Down syndrome. AF from 45 Down syndrome and 150 chromosomally normal pregnancies between 16 and 19 weeks' gestation were analysed, blinded to whether the sample was from a Down syndrome or a normal pregnancy. The median (10th-90th percentiles) inhibin-B level in the control pregnancies increased from 310.0 (80.8-1112.5) pg/ml at 16 weeks to 459.5 (193.7-1386.8) pg/ml at 19 weeks' gestation. The corresponding figures for precursor inhibins (pro-alphaC inhibins) were 541.8 (206.9-1322.8) pg/ml at 16 weeks and 1391.8 (433.3-2652.6) pg/ml at 19 weeks. Expressed as multiples of the median (MOM), the levels of inhibin-B and pro-alphaC inhibins in the Down syndrome samples were 0.85 and 0.79, respectively. Neither was significantly different from the controls. These data suggest that, of the three inhibin subunits, abnormal production or secretion of the inhibin betaA-subunit may underlie the decreased inhibin-A levels previously observed in Down syndrome. Confirmation of this by quantitative assessment of the inhibin subunit messenger ribonucleic acids would now be useful.

Nitric oxide binding and crystallization of recombinant nitrophorin I, a nitric oxide transport protein from the blood-sucking bug Rhodnius prolixus.

A nitric oxide transport protein (nitrophorin I) from the salivary glands of the blood-sucking bug Rhodnius prolixus has been expressed as an insoluble form in Escherichia coli, reconstituted with heme, and characterized with respect to NO binding kinetics and equilibria. NO binding and absorption spectra for recombinant nitrophorin I were indistinguishable from those of the insect-derived protein. The degree of NO binding, the rate of NO release, and the Soret absorption maxima for nitrophorin I were all pH dependent. The NO dissociation constant rose 9-fold over the pH range 5.0-8.3, from 0.19 x 10(-6) to 1.71 x 10(-6). The NO dissociation rate rose 2500-fold between pH 5.0 and pH 8.3, from 1.2 x 10(-3) to 3.0 s(-1). Thus, the NO association rate must also be pH dependent and reduced at pH 5.0 by approximately 280-fold. These factors are consistent with nitrophorin function: NO storage in the apparent low pH of insect salivary glands and NO release into the tissue of the insect's host, where vasodilation is induced. The reversible nature of NO binding, which does not occur with most other heme proteins, and the apparent kinetic control of NO release are discussed. We also report crystals of nitrophorin I that are suitable for structure determination by X-ray crystallography. The most promising crystal form contains two protein molecules in the asymmetric unit and diffracts beyond 2.0 A resolution.

Amplification of 19q13.1-q13.2 sequences in ovarian cancer. G-band, FISH, and molecular studies.

In this study of ovarian carcinoma, we extended previous findings by performing FISH using chromosome 19 paint and microFISH probes and patient samples with and without abnormalities of chromosome 19 identified by G-banding. Karyotype interpretations of der(19) were confirmed, while additional 19 translocations were also detected by FISH with 19WCP in some cases. Similar FISH studies of ovarian carcinoma cell lines found chromosome 19 abnormalities even after extensive in vitro culture. MicroFISH probes were generated by chromosome microdissection from two cases with hsr(19) and mapped to 19q13.2 and 19q13.1-.2, respectively. FISH with these microFISH probes alone or in combination with a 19WCP probe to four patient samples and seven cell lines showed that 65% of chromosome 19 structural abnormalities contained 19q13.1-q13.2 sequences, sometimes as large hsrs. Ovarian cancer cell lines showed amplification and overexpression of the AKT2 putative oncogene, but not the ERCC-2 DNA repair gene in this chromosomal region. In addition to AKT2, amplification and overexpression of other yet-unidentified genes in the 19q13.1-q13.2 region may contribute to ovarian carcinoma pathogenesis or progression.

Analysis of the p53 gene in human precancerous actinic keratosis lesions and squamous cell cancers.

A biomarker of skin cancer would be beneficial in evaluating the efficacy of potential cancer chemoprevention agents. To this end, we investigated the tumor suppressor gene p53 in precancerous actinic keratosis lesions (AK) and malignant squamous cell carcinomas (SCCs) using polymerase chain reaction and single-strand conformation polymorphism analysis (PCR-SSCP) techniques. In addition, p53 protein expression was evaluated using immunohistochemistical analysis with the PAB 1801 monoclonal antibody. Nine out of 13 (69%) SCCs and 8 of 15 (53%) AKs were positive for p53 mutations. In contrast, normal skin samples were negative for p53 mutations. Sequence analysis of AKs and SCCs showed primarily C to T transition mutations. Nuclear immunochemical staining for p53 was observed in 12/15 (80%) AK and 12/13 (92%) SSCs. These results suggest that p53 mutations may be involved in the malignant conversion of AKs to SCCs and that p53 may be useful as a biomarker to study the potential modulatory effects of cancer chemopreventive agents against skin cancer.

Role of residue 478 as a determinant of the substrate specificity of cytochrome P450 2B1.

Two allelic variants and eight site-directed mutants of cytochrome P450 2B1 differing at residue 478 have been expressed in COS cells and assayed for androstenedione hydroxylase activities. The 478Gly and 478Ala variants and five mutants (Ser, Thr, Val, Ile, and Leu) exhibited 16 beta-OH:16 alpha-OH ratios ranging from 0.7 to 9.3, whereas the Pro, Glu, and Arg mutants were expressed but inactive. The seven samples active toward androstenedione also exhibited testosterone 16 beta-OH:16 alpha-OH ratios ranging from 0.4 to 2.3. With both steroids, the Gly variant had the highest 16 beta-hydroxylase activity, and the 16 beta-OH:16 alpha-OH ratio increased with the size of aliphatic size chains (Ala, Val, and Ile/Leu). The highest ratio of androgen 15 alpha:16-hydroxylation was observed with the Ser mutant. On the basis of previous work indicating decreased susceptibility of the 478Ala variant in liver microsomal and reconstituted systems to inactivation by chloramphenicol analogs, methodology was refined for monitoring enzyme inactivation in COS cell microsomes. The Gly and Ala variants were inactivated by chloramphenicol with similar rate constants, whereas the Ser and Val mutants were inactivated more slowly, and the Leu mutant was refractory. Only the Gly variant was inactivated by the chloramphenicol analog N-(2-p-nitrophenethyl)chlorofluoroacetamide. Thus, the side chain of residue 478 appears to be a major determinant of enzyme inactivation as well as of androgen hydroxylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular basis for a functionally unique cytochrome P450IIB1 variant.

Liver microsomes from phenobarbital-treated rats of four inbred strains expressing distinct allelic variants of cytochrome P450IIB1 were analyzed. The Wistar Munich (WM) strain exhibited 5- to 10-fold lower androstenedione 16 beta-hydroxylase activity (a specific P450IIB1 marker) than the Lewis, Wistar Kyoto, and Wistar Furth strains. The androstenedione 16 beta-hydroxylase in the WM liver microsomes was refractory to inactivation by N-(2-p-nitrophenethyl)chlorofluoroacetamide, a selective P450IIB1 inactivator in the other three strains. Purified P450IIB1-WM was insensitive to the inactivator and exhibited 5-fold lower androstenedione 16 beta-hydroxylase, testosterone 16-hydroxylase, and 7-ethoxycoumarin deethylase activities but the same benzphetamine demethylase activity and slightly higher androstenedione 16 alpha-hydroxylase activity than a P450IIB1 purified from outbred Sprague-Dawley rats, which appears to correspond to the form in Lewis rats. The stereoselectivity of androstenedione 16-hydroxylation catalyzed by P450IIB1-WM (16 beta-OH:16 alpha-OH = 1.4) is thus distinct from that (16 beta-OH:16 alpha-OH = 12-15) of other P450IIB1 preparations described. A cDNA encoding P450IIB1-WM was cloned and sequenced, revealing a single amino acid substitution (Gly-478----Ala) compared with the published sequence (Fujii-Kuriyama, Y., Mizukami, Y., Kawajiri, K., Sogawa, K., and Muramatsu, M. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2793-2797). Heterologous expression of P450IIB1 and P450IIB1-WM confirmed the striking difference in androstenedione metabolite profiles, strongly implicating the involvement of Ala-478 in defining the distinctive catalytic properties of P450IIB1-WM.

Interhemispheric differences in the planning and execution of sequences of skilled finger movements.

An index-middle finger (double) tapping task was used to examine hemispheric differences in the planning and execution of skilled finger movements. In two experiments, subjects responded to a simple cue presented tachistoscopically in the left or right visual field, by performing a predetermined number of double taps, (between one and eight inclusive), with either the left or right hand. Reaction times (RT) increased linearly as a function of increasing number of taps, when response sequences were controlled by the left hemisphere. In contrast, an inverse quadratic trend was obtained with right hemisphere control. When both hemispheres were involved in the stimulus-response sequence, the latency function incorporated elements of both trends, suggesting interaction between the hemispheres. The RT trends reflect differences in motor planning between the hemispheres. The conditions engaging only the right or left hemispheres did not differ in motor execution, as measured by tapping duration, variability or errors. However, when both hemispheres were involved there was evidence of interaction, which was evident as interference when the right visual field or left hemisphere was cued but the motor response was under the control of the right hemisphere (left hand). Overall, the results indicate that hand differences in fine motor skill may be determined by hemispheric differences associated with motor preparation rather than response execution.

Selective inactivation by chlorofluoroacetamides of the major phenobarbital-inducible form(s) of rat liver cytochrome P-450.

Five N-monosubstituted chlorofluoroacetamides have been tested as potential specific irreversible inhibitors of the major phenobarbital-inducible form of rat liver cytochrome P-450 (P450IIB1). In vitro, N-(2-phenethyl)chlorofluoroacetamide was ineffective in causing a time-dependent loss of P450IIB1-mediated androstenedione 16 beta-hydroxylase activity in liver microsomes from phenobarbital-treated rats. However, addition of a nitro or bromo substitutent at the para position of the phenyl group or addition of a second phenyl group at the 1- or 2-position on the phenethyl side chain yielded compounds that caused a selective time-dependent decrease in androstenedione 16 beta-hydroxylase activity relative to four other P-450 form-specific androstenedione or progesterone hydroxylase activities monitored. The two compounds that were the most effective in inactivating P450IIB1 in vitro, N-(2-p-bromophenethyl) and N-(2-p-nitrophenethyl)chlorofluoroacetamide were also administered ip to phenobarbital-treated rats, and inhibition of cytochromes P-450 was assessed by in vitro assays of steroid and R- and S-warfarin hydroxylation in subsequently prepared hepatic microsomes. Both compounds selectively inhibited P450IIB1, and at a dose (200 mg/kg) of N-(2-p-nitrophenethyl)chlorofluoroacetamide that reduced androstenedione 16 beta-hydroxylase activity to approximately one-third of the control level, only two other activities, both attributable to P450IIB1, were decreased. In contrast, steroid and warfarin hydroxylase activities indicative of at least five other cytochromes P-450 were unaffected by the compound. These results indicate the feasibility of an empirical approach to the design of specific cytochrome P-450 inactivators.

Selective inactivation of rat liver cytochromes P-450 by 21-chlorinated steroids.

The inactivation by 21-chlorinated steroids of rat liver cytochromes P-450 involved in the hydroxylation of progesterone and androstenedione has been investigated. Preincubation of intact liver microsomes from phenobarbital-treated rats with 21-chloropregnenolone, 21,21-dichloropregnenolone, or 21,21-dichloroprogesterone in the presence of NADPH caused a time-dependent decrease in progesterone 21-hydroxylase and in progesterone or androstenedione 6 beta-hydroxylase activity but had negligible or only minor effects on five other steroid hydroxylases. The compounds differed, however, with regard to the relative rate constants for inactivation of the 21- and 6 beta-hydroxylases. For example, 21,21-dichloroprogesterone and 21,21-dichloropregnenolone inactivated the progesterone 6 beta-hydroxylase at similar rates, but the dichloroprogesterone was a more effective inactivator of the 21-hydroxylase. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few isozymes of cytochrome P-450 may be a rational means of designing isozyme-selective inhibitors but that target and nontarget enzymes may not totally retain the regioselectivity they exhibit towards the underivatized substrate.

Specific inactivation by 17 beta-substituted steroids of rabbit and rat liver cytochromes P-450 responsible for progesterone 21-hydroxylation.

The selective inactivation by 17 beta-substituted steroids of rabbit and rat liver cytochromes P-450 involved in the 21-hydroxylation of progesterone has been investigated. Five derivatives each of pregnenolone and progesterone were prepared, in which the methylketo substituent of the 17 beta-position was replaced by a dichloromethylketo, chlorofluoromethylketo, difluoromethylketo, vinyl, or ethynyl group. The ability of the compounds to cause time-dependent (inactivation) and time-independent (inhibition) decreases in progesterone hydroxylase activity was assessed in vitro using intact liver microsomes as well as reconstituted systems containing the major forms of hepatic cytochrome P-450 responsible for progesterone 21-hydroxylation, P-450 1 in the rabbit and PB-C in the rat. In each species, one compound was identified that specifically inactivated the 21-hydroxylase, namely 21-chloro-21-fluoropregnenolone in the rabbit and pregn-4,20-diene-3-one in the rat, although both compounds inhibited several other hydroxylases as well. Moreover, the most effective and specific 21-hydroxylase inactivators were not necessarily the most effective or specific inhibitors. These results suggest that conversion of the enzyme-inhibitor complex to metabolites that inactivate the enzyme, rather than complex formation, is the crucial factor in determining the specificity of the compounds as cytochrome P-450 inactivators. The results indicate the feasibility of designing specific inactivators of hepatic cytochromes P-450 by utilizing the normal regioselectivity of the target enzyme towards steroids.

Selective inactivation by 21-chlorinated steroids of rabbit liver and adrenal microsomal cytochromes P-450 involved in progesterone hydroxylation.

The inactivation by 21-chlorinated steroids of rabbit liver cytochromes P-450 involved in the hydroxylation of progesterone has been investigated in intact microsomes encompassing two phenotypes of 21-hydroxylase activity, two phenotypes of 16 alpha-hydroxylase activity, and three phenotypes of 6 beta-hydroxylase activity. In liver microsomes from outbred New Zealand White male rabbits exhibiting a high content of cytochrome P-450 1, 21,21-dichloropregnenolone caused a time- and NADPH-dependent loss of 21-hydroxylase activity. This loss of activity exhibited a number of characteristics of mechanism-based inactivation, including irreversibility, saturation with increasing inhibitor concentrations, and protection by substrate, and was also documented with purified P-450 1 in a reconstituted system. 21,21-Dichloropregnenolone caused no time-dependent loss of 6 beta-hydroxylase activity in microsomes from the New Zealand White rabbits or from control or rifampicin-treated rabbits of the inbred B/J strain. In contrast, in the microsomes from the B/J rabbits, some inactivation of the 16 alpha-hydroxylase was observed (k = 0.04 min-1), regardless of the rifampicin treatment. The other two compounds tested, 21-chloropregnenolone and 21,21-dichloroprogesterone, were less effective than the dichloropregnenolone as inactivators of cytochrome P-450 1. On the other hand, 21,21-dichloroprogesterone, but not 21,21-dichloropregneolone, caused a rapid time-dependent loss of 21-hydroxylase activity in rabbit adrenal microsomes. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few forms of cytochrome P-450 may be a rational means of designing selective inhibitors of the enzyme.