Increased hepatic mitochondrial capacity in rats with hydroxy-cobalamin[c-lactam]-induced methylmalonic aciduria.

Abstract

IntroductionTreatment of rats with the vitamin B12 analogue hydroxy-cobalaminic-lactaml (HCCL) impairs methylmalonyl-CoA mutase function and leads to methylmalonic aciduria due to intracellular accumulation of propionyl and methylmalonyl-CoA.Since accumulation of these acyl-CoAs disrupts normal cellular regulation, the present investigation characterized metabolism in hepatocytes and liver mitochondria from rats treated subcutaneously with HCCL or saline (control) by osmotic minipump. Consistent with decreased methylmalonyl-CoA mutase activity, 14C02 production from 1-"4C-propionate (1 mM) was decreased by 76% and 82% after 2-3 wk and 5-6 wk of HCCL treatment, respectively. In contrast, after 5-6 wk of HCCL treatment, "4CO2 production from 1-"4C-pyruvate (10 mM) and 1-'4C-palmitate (0. Vitamin B 12 (cobalamin) deficiency leads to decreased activity of the cobalamin-requiring enzymes methylmalonyl-CoA mutase and methionine synthetase (1). Decreased activity of methylmalonyl-CoA mutase results in accumulation of methylmalonyl-and propionyl-CoA, and methylmalonic aciduria (2-4). This defect is analogous to the metabolic defect in patients with hereditary methylmalonic acidurias. The vitamin B 12-deficient rat has provided a useful animal model for the human methylmalonic acidurias and allowed the investigation of hepatic metabolism in the presence of this specific, metabolic insult (4-6). Dietary vitamin B 12 deficiency is difficult to achieve, and the resulting metabolic defect shows a high variability between treated animals (7, 8). An alternative methodology to induce decreased activity of methylmalonyl-CoA mutase is treatment with hydroxy-cobalamin [c-lactam] (HCCL),' a synthetic vitamin B 12 analogue (4-6). Rats treated with HCCL develop decreased hepatic vitamin B 12 levels (4), decreased methylmalonyl-CoA mutase and methionine synthetase activity (6), and decreased propionate metabolism with an increased renal excretion of methylmalonic acid (4-6).Increased hepatocellular propionyl and methylmalonylCoA concentrations inhibit several metabolic activities, including gluconeogenesis (9, 10), urea synthesis (9, 11) fatty acid oxidation (11, 12) and pyruvate decarboxylation (13, 14). In contrast, in rats with chronic accumulation of propionyl and methylmalonyl-CoA induced by dietary vitamin B 12 deficiency, gluconeogenetic and ketogenetic responses to fasting were intact in vivo (4), suggesting compensatory mechanisms that mitigate the toxic effects of accumulated acyl-CoAs. One compensatory mechanism in rats with dietary and functional vitamin B 12 deficiency is increased hepatic total CoA concentration (4, 15).To further study hepatic metabolism under conditions of impaired methylmalonyl-CoA mutase activity, hepatic mitochondrial function in HCCL-and saline-treated rats was characterized. Mitochondrial function was quantified by measurement of fuel oxidation rates in isolated hepatocytes and mitochondria. In addition, activities of mitochondrial and extramitochondrial enzymes were determined in isolat...

Rat liver mitochondrial proteome: Changes associated with aging and acetyl-L-carnitine treatment

“…ALCAR decreased as well the level of a specific enzyme of ketone body biosynthesis but activated the acetylCoA acetyltransferase. It is interesting to note that the only one enzyme of lipid and ketone body metabolism whose increased content is reversed by ALCAR treatment is methylmalonil-CoA mutase, an enzyme involved not only in beta oxidation of fatty acids but also in degradation of several aminoacids and of cholesterol via propionyl CoA [51].…”

“…Furthermore ALCAR prevented the increase of the methylmalonyl-CoA mutase (Fig. 2G) an enzyme involved in several metabolism [51].…”

Translation rates of isolated liver mitochondria under conditions of hepatic mitochondrial proliferation

“…The [16]) and may reflect a mitochondrial translation capacity intrinsic to the liver mitochondria. Thus, while decreases in the normal hepatic mitochondrial content are associated with decreases in mitochondrial translation, increases from the normal as a result of hyperthyroidism [13] or HCCL or clofibrate treatment are not.…”

“…Effects of treatments were determined by Student's unpaired t test, with P < 0.05 considered significant. models in previous studies with a similar duration of treatment [16,19,27]. Interpretation of radiolabelled amino acid incorporation rates is potentially complicated by changes in endogenous amino acid content in the isolated mitochondria following the treatments of interest.…”

“…Administration of the hypolipidaemic drug clofibrate is associated with increased contents of liver mitochondria and peroxisomes, via, unknown mechanisms [15]. Recently we have described hepatic niitochondrial proliferation in an animal model of methylmalonic aciduria induced by administration of the cobalamin analogue hydroxycobalamin[c-lactam] (HCCL) [16,17]. Translation rates were quantified in liver mitochondria isolated from control, clofibrate-and HCCL-treated rats to determine if the treatments modified mitochondrial protein expression.…”