MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement.

MSTO1 encodes a cytosolic mitochondrial fusion protein, misato homolog 1 or MSTO1. While the full genotype-phenotype spectrum remains to be explored, pathogenic variants in MSTO1 have recently been reported in a small number of patients presenting with a phenotype of cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic and pigmentary retinopathy. The proposed underlying pathogenic mechanism of MSTO1-related disease is suggestive of impaired mitochondrial fusion secondary to a loss of function of MSTO1. Disorders of mitochondrial fusion and fission have been shown to also lead to mitochondrial DNA (mtDNA) depletion, linking them to the mtDNA depletion syndromes, a clinically and genetically diverse class of mitochondrial diseases characterized by a reduction of cellular mtDNA content. However, the consequences of pathogenic variants in MSTO1 on mtDNA maintenance remain poorly understood. We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants, and we provide functional characterization from seven MSTO1-related disease patient fibroblasts. Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy. MSTO1 protein was not detectable in the cultured fibroblasts of all seven patients evaluated, suggesting that pathogenic variants result in a loss of protein expression and/or affect protein stability. Consistent with impaired mitochondrial fusion, mitochondrial networks in fibroblasts were found to be fragmented. Furthermore, all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids. Our data corroborate the role of MSTO1 as a mitochondrial fusion protein and highlight a previously unrecognized link to mtDNA regulation. As impaired mitochondrial fusion is a recognized cause of mtDNA depletion syndromes, this novel link to mtDNA depletion in patient fibroblasts suggests that MSTO1-deficiency should also be considered a mtDNA depletion syndrome. Thus, we provide mechanistic insight into the disease pathogenesis associated with MSTO1 mutations and further define the clinical spectrum and the natural history of MSTO1-related disease.

NDUFA9 point mutations cause a variable mitochondrial complex I assembly defect.

Mitochondrial respiratory chain complex I consists of 44 different subunits and contains 3 functional modules: the Q-, the N- and the P-module. NDUFA9 is a Q-module subunit required for complex I assembly or stability. However, its role in complex I biogenesis has not been studied in patient fibroblasts. So far, a single patient carrying an NDUFA9 variant with a severe neonatally fatal phenotype has been reported. Via exome sequencing, we identified a novel homozygous NDUFA9 missense variant in another patient with a milder phenotype including childhood-onset progressive generalized dystonia and axonal peripheral neuropathy. We performed complex I assembly analysis using primary skin fibroblasts of both patients. Reduced complex I abundance and an accumulation of Q-module subassemblies were present in both patients but more pronounced in the severe clinical phenotype patient. The latter displayed additional accumulation of P-module subassemblies, which was not present in the milder-phenotype patient. Lentiviral complementation of both patient fibroblast cell lines with wild-type NDUFA9 rescued complex I deficiency and the assembly defects. Our report further characterizes the phenotypic spectrum of NDUFA9 deficiency and demonstrates that the severity of the clinical phenotype correlates with the severity of the effects of the different NDUFA9 variants on complex I assembly.

Population-specific recombination sites within the human MHC region.

Genetic rearrangement by recombination is one of the major driving forces for genome evolution, and recombination is known to occur in non-random, discreet recombination sites within the genome. Mapping of recombination sites has proved to be difficult, particularly, in the human MHC region that is complicated by both population variation and highly polymorphic HLA genes. To overcome these problems, HLA-typed individuals from three representative populations: Asian, European and African were used to generate phased HLA haplotypes. Extended haplotype homozygosity (EHH) plots constructed from the phased haplotype data revealed discreet EHH drops corresponding to recombination events and these signatures were observed to be different for each population. Surprisingly, the majority of recombination sites detected are unique to each population, rather than being common. Unique recombination sites account for 56.8% (21/37 of total sites) in the Asian cohort, 50.0% (15/30 sites) in Europeans and 63.2% (24/38 sites) in Africans. Validation carried out at a known sperm typing recombination site of 45 kb (HLA-F-telomeric) showed that EHH was an efficient method to narrow the recombination region to 826 bp, and this was further refined to 660 bp by resequencing. This approach significantly enhanced mapping of the genomic architecture within the human MHC, and will be useful in studies to identify disease risk genes.

Pain Relief After Total Knee Arthroplasty with Intravenous and Periarticular Corticosteroid: A Randomized Controlled Trial.

BACKGROUND: Total knee arthroplasty (TKA) is a cost-effective procedure, but it is also associated with substantial postoperative pain. The present study aimed to compare pain relief and functional recovery after TKA among groups that received intravenous corticosteroids, periarticular corticosteroids, or a combination of both. METHODS: This randomized, double-blinded clinical trial in a local institution in Hong Kong recruited 178 patients who underwent primary unilateral TKA. Six of these patients were excluded because of changes in surgical technique; 4, because of their hepatitis B status; 2, because of a history of peptic ulcer; and 2, because they declined to participate in the study. Patients were randomized 1:1:1:1 to receive placebo (P), intravenous corticosteroids (IVS), periarticular corticosteroids (PAS), or a combination of intravenous and periarticular corticosteroids (IVSPAS). RESULTS: The pain scores at rest were significantly lower in the IVSPAS group than in the P group over the first 48 hours (p = 0.034) and 72 hours (p = 0.043) postoperatively. The pain scores during movement were also significantly lower in the IVS and IVSPAS groups than in the P group over the first 24, 48, and 72 hours (p ≤ 0.023 for all). The flexion range of the operatively treated knee was significantly better in the IVSPAS group than in the P group on postoperative day 3 (p = 0.027). Quadriceps power was also greater in the IVSPAS group than in the P group on postoperative days 2 (p = 0.005) and 3 (p = 0.007). Patients in the IVSPAS group were able to walk significantly further than patients in the P group in the first 3 postoperative days (p ≤ 0.003). Patients in the IVSPAS group also had a higher score on the Elderly Mobility Scale than those in the P group (p = 0.036). CONCLUSIONS: IVS and IVSPAS yielded similar pain relief, but IVSPAS yielded a larger number of rehabilitation parameters that were significantly better than those in the P group. This study provides new insights into pain management and postoperative rehabilitation following TKA. LEVEL OF EVIDENCE: Therapeutic Level I . See Instructions for Authors for a complete description of levels of evidence.

B*13:50, a novel HLA-B*13 allele, identified by sequence-based typing.

We report a novel HLA-B*13 allele, B*13:50, found using high-resolution sequence-based typing in a Chinese donor. B*13:50 differs from B*13:01:01 by a single-nucleotide substitution (A→T) at position 482, in exon 3.

Identification of a novel HLA-C allele, HLA-C*03:85, in a Singaporean Chinese.

HLA-C*03:85 differs from C*03:03:01 by a single nucleotide substitution at position 276, in exon 2.

Production of natural killer cell stimulatory factor (interleukin 12) by peripheral blood mononuclear cells.

Natural killer cell stimulatory factor (NKSF), or interleukin 12 (IL-12), is a 70-kD heterodimeric cytokine composed of two covalently linked chains, p40 and p35. NKSF/IL-12 has multiple effects on T and NK cells and was originally identified and purified from the supernatant fluid of Epstein-Barr virus (EBV)-transformed human B lymphoblastoid cell lines. We have produced a panel of monoclonal antibodies against both chains of NKSF/IL-12. Some of these antibodies have neutralizing activity, and several combinations of them have been used to establish sensitive radioimmunoassays detecting the free p40 chain, the free p35 chain, or the p70 heterodimer. Using these reagents, we have determined that most EBV-transformed human B lymphoblastoid cell lines constitutively produce low levels of the p70 heterodimer and an excess of the free p40 chain, whereas Burkitt lymphoma-derived, T, myeloid, and many solid tumor-derived cell lines produce neither. Production of both p40 and p70 is increased several-fold upon stimulation of the EBV-transformed cell lines with phorbol diesters. The ability of supernatant fluids from unstimulated and phorbol diester-stimulated cell lines to induce interferon gamma (IFN-gamma) production from T and NK cells, one of the effects of NKSF/IL-12, parallels the levels of production of the p70 heterodimer, known to be the biologically active form of NKSF/IL-12. Staphylococcus aureus Cowan I strain (SAC) and other stimuli induce accumulation of p40 mRNA and production of both p40 and p70 by peripheral blood mononuclear cells (PBMC). The producer cells appear to include both adherent cells and nonadherent lymphocytes, possibly B cells. The supernatant fluids from SAC-stimulated PBMC mediate the typical functions of NKSF/IL-12 (i.e., IFN-gamma induction, mitogenic effects on T/NK blasts, enhancement of NK cell cytotoxicity) at concentrations of p70 similar to those at which recombinant NKSF/IL-12 mediates the same functions. Moreover, these activities are significantly inhibited by anti-NKSF/IL-12 antibodies. The neutralizing anti-NKSF/IL-12 antibodies also inhibit 85% of the IFN-gamma production in response to SAC, an NKSF/IL-12 inducer, and approximately 50% of the IFN-gamma production in response to non-NKSF/IL-12-inducers such as IL-2, phytohemagglutinin, and anti-CD3 antibodies. These results indicate that induced or constitutively produced NKSF/IL-12 has a major role in facilitating IFN-gamma production by peripheral blood lymphocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of interferon gamma production by natural killer cell stimulatory factor: characterization of the responder cells and synergy with other inducers.

We previously reported that natural killer cell stimulatory factor (NKSF), a heterodimeric lymphokine purified from the conditioned medium of human B lymphoblastoid cell lines, induces interferon gamma (IFN-gamma) production from resting peripheral blood lymphocytes (PBL) and synergizes with interleukin 2 in this activity. In this study, we show that human NKSF induces IFN-gamma production from both resting and activated human PBL and from freshly isolated murine splenocytes. Human T and NK cells produce IFN-gamma in response to NKSF, but resting PBL require the presence of nonadherent human histocompatibility leukocyte antigens DR+ (HLA-DR+) accessory cells to respond to NKSF. The mechanism(s) by which NKSF induces IFN-gamma production results in accumulation of IFN-gamma mRNA, is insensitive to cyclosporin A, and synergizes with those mediated by phytohemagglutinin, phorbol diesters, anti-CD3 antibodies, and allogeneic antigens, but not by Ca2+ ionophores. The ability of NKSF to directly induce IFN-gamma production and to synergize with other physiological IFN-gamma inducers, joined with the previously described ability to enhance lymphocyte cytotoxicity and proliferation, indicates that this lymphokine is a powerful immunopotentiating agent.

Upregulation of nitric oxide synthase II contributes to apoptotic cell death in the hippocampal CA3 subfield via a cytochrome c/caspase-3 signaling cascade following induction of experimental temporal lobe status epilepticus in the rat.

Status epilepticus results in preferential neuronal cell loss in the hippocampus. We evaluated the hypothesis that the repertoire of intracellular events in the vulnerable hippocampal CA3 subfield after induction of experimental temporal lobe status epilepticus entails upregulation of nitric oxide synthase II (NOS II), followed by the release of mitochondrial cytochrome c that triggers the cytosolic caspase-3 cascade, leading to apoptotic cell death. In Sprague-Dawley rats, significant and temporally correlated upregulation of NOS II (3-24h), but not NOS I or II expression, enhanced cytosolic translocation of cytochrome c (days 1 and 3), augmented activated caspase-3 in cytosol (days 1, 3 and 7) and DNA fragmentation (days 1, 3 and 7) was detected bilaterally in the hippocampal CA3 subfield after elicitation of sustained seizure activity by microinjection of kainic acid into the unilateral CA3 subfield. Application bilaterally into the hippocampal CA3 subfield of a selective NOS II inhibitor, S-methylisothiourea, significantly blunted these apoptotic events; a selective NOS I inhibitor, N(omega)-propyl-l-arginine or a potent NOS III inhibitor, N(5)-(1-iminoethyl)-l-ornithine was ineffective. We conclude that upregulation of NOS II contributes to apoptotic cell death in the hippocampal CA3 subfield via a cytochrome c/caspase-3 signaling cascade following the induction of experimental temporal lobe status epilepticus.

Cholinergic-receptor-independent dysfunction of mitochondrial respiratory chain enzymes, reduced mitochondrial transmembrane potential and ATP depletion underlie necrotic cell death induced by the organophosphate poison mevinphos.

Our current understanding of the nature of cell death that is associated with fatal organophosphate poisoning and the underlying cellular mechanisms is surprisingly limited. Taking advantage of the absence in an in vitro system of acetylcholinesterase, the pharmacological target of organophosphate compounds, the present study evaluated the hypothesis that the repertoire of cholinergic receptor-independent cellular events that underlie fatal organophosphate poisoning entails induction of mitochondrial dysfunction, followed by bioenergetic failure that leads to necrotic cell death because of ATP depletion. Pheochromocytoma PC12 cells incubated with the organophosphate pesticide mevinphos (0.4 or 4mumol) for 1 or 3h underwent a dose-related and time-dependent loss of cell viability that was not reversed by muscarinic (atropine) or nicotinic (mecamylamine) blockade. This was accompanied by depressed NADH cytochrome c reductase, succinate cytochrome c reductase or cytochrome c oxidase activity in the mitochondrial respiratory chain, reduced mitochondrial transmembrane potential, decreased ATP concentration, elevated ADP/ATP ratio, increased lactate dehydrogenase release and necrotic cell death. We conclude that Mev induces cholinergic receptor-independent necrotic cell death by depressing the activity of Complexes I to IV in the mitochondrial respiratory chain, eliciting reduction in mitochondrial transmembrane potential, depleting intracellular ATP contents and damaging cell membrane integrity.

Inhibition of lymphocyte proliferation by sera from patients with hepatocellular carcinoma: lack of correlation with serum alpha-fetoprotein levels.

Sera from 23 hepatocellular carcinoma (HCC) patients were studied for their effects on lymphocyte proliferation during polyclonal mitogen stimulation and one-way mixed lymphocyte culture (MLC). The effect of 25% HCC serum on tritiated thymidine uptake of cultures varied from slight augmentation to 95% inhibition; however, no relationship was found between the potency of inhibition and serum alpha-fetoprotein (AFP) concentration. Purified AFP from the ascitic fluid of an HCC patient had no effect on MLC or cultures containing pokeweed mitogen; but at a concentration of 200 micro g/ml, AFP inhibited up to 30% of the proliferative responses in cultures containing phytohemagglutinin or concanavalin A. Results strongly suggest that the AFP in HCC sera is not inhibitory in itself.

HLA antigen in Chinese patients with hepatocellular carcinomas.

One hundred and fourteen Chinese patients with hepatocellular carcinomas (HCC) were studied for alpha-fetoprotein (AFP), hepatitis B surface antigen (HBsAg), anti-HBsAg, and HLA markers. Younger patients with HCC (less than 60 yr old) were more significantly associated with elevated serum AFP (P less than 0.0001) and serum HBsAg (P less than 0.0001) than were older (greater than or equal to 60 yr old) patients. A strong correlation existed between serum AFP and HBsAg among the patients (P less than 0.0001). Of 27 AFP-negative patients, 15 (55.6%) had HLA-B15 compared to 53 of 238 (22.3%) healthy controls (corrected P less than 0.003, relative risk = 4.4). The frequency of HLA-B15 was even higher in the AFP-negative plus HBsAg-negative subgroup (61%). AFP-negative patients also had a significant lack of blood group A.

Immunogenetic aspects of nasopharyngeal carcinoma. IV. Increased risk in Chinese of nasopharyngeal carcinoma associated with a Chinese-related HLA profile (A2, Singapore 2).

The results of this study of 110 Singapore Chinese with nasopharyngeal carcinoma (NPC) and 91 controls confirmed the association between the occurrence of HLA antigen Singapore 2 (Sin2) and NPC in the Chinese population, and indicated that their increased risk for NPC was confined to the joint occurrence of Sin 2 and A2 antigens. These findings suggested that the genotype of importance in susceptibility to NPC is the A2-Sin 2 haplotype.

MicroAlbuminuria Prevalence Study (MAPS) in hypertensive type 2 diabetic patients in Hong Kong.

OBJECTIVES: To assess the prevalence of macroalbuminuria and microalbuminuria, and the level of blood pressure control in patients with type 2 diabetes and hypertension in Hong Kong. DESIGN: Cross-sectional clinic-based epidemiological study. SETTING: Six medical centres (including two public hospital diabetes centres) in Hong Kong. PATIENTS: Recruited from the medical centres from April to November 2002, after excluding those with bacteriuria and haematuria. MAIN OUTCOME MEASURES: Body mass index; blood pressure; levels of blood glucose, macroalbuminuria, and microalbuminuria; treatments for hypertension and diabetes. RESULTS: The as per-protocol recruited population of 437 hypertensive type 2 diabetic patients had a mean age of 61.7 (standard error, 0.5) years. Overall, the prevalence of diabetic nephropathy in this population was high; 18.3% had macroalbuminuria (95% confidence interval, 16.5-20.2%) and 24.9% had microalbuminuria (95% confidence interval, 22.9-27.0%). Predictive factors were advanced age, male sex, poor blood pressure control, and existing cardiovascular complications. Whilst almost all patients (96.1%) were receiving treatment for hypertension, only 25.6% had systolic/diastolic blood pressures below the 130/85 mm Hg target. CONCLUSIONS: In Hong Kong, the prevalence of microalbuminuria and macroalbuminuria is high in type 2 diabetic patients with hypertension, particularly in males and those with poorly controlled systolic blood pressure. Tight glycaemic control, antihypertensive therapy, and use of renin-angiotensin system inhibitors/blockers are necessary to retard the progression of nephropathy to advanced renal disease.

Immunogenicity and reactogenicity of a reduced-antigen-content diphtheria-tetanus-acellular pertussis vaccine as a single-dose booster in Singaporean adults.

INTRODUCTION: Older children and adults, susceptible to pertussis because of waning immunity, may serve as a reservoir of infection, leading to severe disease among young unvaccinated infants. Booster diphtheria-tetanus-acellular pertussis (dTpa) vaccination in older age groups is rare in Singapore, one reason being the increase in reactogenicity with each successive dose. The aim of this study was to assess the immunogenicity, safety and reactogenicity of a reduced antigen, combined dTpa vaccine as a single booster dose in healthy adults aged 18 years or older. METHODS: A total of 150 healthy adults, 18 to 60 years of age, received a single dose of GlaxoSmithKline Biologicals' dTpa vaccine with reduced content for diphtheria and pertussis, with measurement of pre- and post-vaccination antibody titres. RESULTS: Prior to vaccination, 71.6 percent and 92.6 percent of the subjects had anti-diphtheria and anti-tetanus antibody levels greater than or equal to 0.1 IU/mL, respectively. 46.7 percent, 98.5 percent and 44.4 percent of subjects were seropositive for pertussis toxin (PT), filamentous haemagglutinin (FHA) and pertactin (PRN) antibodies, respectively. One month after vaccination, there was an increase in geometric mean titres from pre-vaccination to post-vaccination blood samples for anti-diphtheria (greater than seven-fold), anti-tetanus (greater than five-fold), anti-PT (greater than 11-fold), anti- FHA (greater than 25-fold) and anti-PRN (greater than 31-fold) antibodies. Solicited grade three local symptoms (pain, redness and swelling) were reported in 14.1 percent, 8.1 percent and 10.4 percent of subjects, respectively. No serious adverse events were reported. CONCLUSION: In summary, the dTpa vaccine is immunogenic, safe and well-tolerated in Singaporean adults.

Adenine nucleotide transport in hepatoma mitochondria and its correlation with hepatoma growth rates and tumor size.

Initial rates of [3H]adenosine diphosphate and [3H]adenosine triphosphate uptake were measured in mitochondria isolated from normal rat liver, regenerating liver, mouse hepatoma BW7756, and four Morris hepatomas (7777, 7800, 7794A, and 16) of varying degrees of malignancy. Results obtained demonstrate that (a) the apparent Km and Vmax values for adenosine diphosphate and adenosine triphosphate uptake are significantly lower in hepatoma compared to normal or regenerating liver mitochondria, (b) the Vmax values for adenosine diphosphate uptake correlate with tumor growth rate, and (c) the Km values for adenosine triphosphate in both hepatoma and normal mitochondria are lowered in the presence of added uncoupling agents; however, the extent of decrease is much less in fast-growing tumors than in slow-growing tumors and normal tissues. Studies examining the causes of reduced transport rates in hepatoma mitochondria showed that they are independent of the mitochondrial energy state and associated with substantially lower levels of the total and exchangeable adenine nucleotides. Additional studies revealed that transport rates are also dependent on the size of the tumor from which the mitochondria are isolated. Mitochondria isolated from small tumors (less than 2 g) had higher transport rates as well as higher levels of exchangeable and total adenine nucleotides than those isolated from larger tumors (4 to 6 g). Endogenous inhibitor levels also varied as a function of tumor size; free fatty acid levels increased, whereas acyl coenzyme A levels declined in mitochondria isolated from larger tumors. These results seem to indicate that, during the progression of tumor growth, mitochondria are experiencing cellular environmental changes that will affect overall tumor cell metabolism.

Efflux of adenine nucleotides in mitochondria from rat tumor cells of varying growth rates.

The efflux of adenine nucleotides was studied in mitochondria isolated from normal rat liver, host livers, and the tumors from four Morris hepatoma lines of varying growth rates. [3H]Adenosine diphosphate (ADP) or [3H]adenosine triphosphate (ATP) was preloaded to the energized mitochondria, and the initial rates of exchange with unlabeled extramitochondrial nucleotides were measured with the carboxyatractyloside stop method. Results indicate that the Vmax values of ATP efflux in mitochondria from fast and intermediately growing tumors (hepatoma cell lines 7777, 7800, and 5123D) are significantly smaller than that of host or normal liver mitochondria, while in slow growing tumor (line 16) the Vmax is not different. On the other hand, for ADP efflux, the opposite (namely, higher in tumor than in host) is observed in the mitochondria of fast growing tumors. Preincubation with the divalent cation ionophore A23187 and calcium chelator ethyleneglycolbis(beta-aminoethyl ether)-N,N'-tetraacetic acid increases the efflux of both ATP and ADP (to a lesser extent) in these tumor mitochondria, indicating that the extraordinarily high concentrations of calcium form complexes with adenine nucleotides (particularly ATP) and thus lower the effective concentrations of free nucleotides for translocation. Together with previously published results (R. L. Barbour and S. H. P. Chan, Cancer Res., 43: 1511-1517, 1983) on lower nucleotide uptake rates in these tumor mitochondria, we propose that the lower ATP efflux and higher ADP efflux rates may cause a futile cycle of ADP transport across the mitochondrial membrane which may contribute to high rates of aerobic glycolysis (by stimulating key glycolytic enzymes such as hexokinase and phosphofructokinase) observed in these fast and intermediately growing tumors.

Adenine nucleotide transport in hepatoma mitochondria. Characterization of factors influencing the kinetics of ADP and ATP uptake.

Initial velocity measurements of [3H]ADP and [3H]ATP uptake have been made with mitochondria isolated from Morris hepatomas of differing growth rates, and factors known to influence the rates of nucleotide exchange have been examined in an effort to determine whether the elevated rates of aerobic glycolysis in these tumors can be attributed to altered carrier activity. These studies included the determination of the apparent kinetic constants for nucleotide uptake as a function of the mitochondrial energy state and the dependence of transport rates on temperature. Also included in these studies were measurements of the mitochondrial levels of endogenous inhibitors, divalent cations and internal adenine nucleotides. Results obtained showed that with mitochondria isolated from the various tumor lines, the apparent kinetic constants for nucleotide uptake are different from those of control rat or regenerating liver mitochondria; the apparent Vmax values for both ADP and ADP uptake are significantly lower. Furthermore, under conditions of a high-energy state, the Km and Vmax values for ATP uptake are greater than the Km and Vmax value for ADP uptake but that under uncoupled conditions, the opposite is observed. Comparison of the levels of mitochondrial Ca2+, Mg2+, long-chain acyl-CoA ester and adenine nucleotide from the various mitochondria showed that important differences exist between liver and hepatoma mitochondria in the levels of Ca2+, long-chain acyl-CoA ester and AMP. Mitochondrial Ca2+ levels are elevated 3-5--fold in all tumor lines, and for Morris 7777 hepatoma (a rapidly growing tumor) by a remarkable 70-fold; whereas the levels of acyl-CoA ester and AMP are significantly lower in the more rapidly growing tumors. Arrhenius plots for nucleotide uptake in mitochondria from liver and hepatoma are characterized as being biphasic, having similar activation energies above and below the break point temperature (28-38 and 6-16 kcal/mol, respectively). However, the transition temperature for mitochondria from the various hepatomas is uniformly 4-5 degrees C lower than mitochondria from control liver. The latter difference may reflect a variation in membrane composition, most probably lipid components. It is concluded that the presence of elevated levels of Ca2+ and lower levels of AMP in hepatoma mitochondria and difference of membrane compositions may play an important role in limiting adenine nucleotide transport activity in vivo and that the impaired carrier activity may contribute to higher rates of aerobic glycolysis observed in these tumors.