To screen fibroblasts for defects in lactate/pyruvate oxidation, cells were grown to confluence in 25-cm2 flasks, rinsed, and incubated in glucose-free media containing 25 microM L-lactate and 0.1 microCi [D,L-1-14C]lactate. Lactate oxidation was measured as the amount of lactate oxidized in nmol of 14CO2 generated/mg protein/min. Fibroblasts from patients with mitochondrial or peroxisomal disorders had decreased lactate oxidation compared to the control (CON): CON, 1.9 +/- 0.13 nmol/mg/min; neonatal adrenoleukodystrophy (NALD), 0.45 +/- 0.01 (P < 0.001); rhizomelic chondrodysplasia punctata (RCDP), 0.13 +/- 0.002 (P < 0.001); mitochondrial defect of unknown etiology (MIT), 0.77 +/- 0.003 (P < 0.001); pyruvate dehydrogenase (PDH) deficiency, 0.98 +/- 0.02 (P < 0.001). This method is useful for screening fibroblasts for defects in lactate oxidation in patients with mitochondrial or peroxisomal disorders. Confirmation of the site of the defect may then be investigated with specific assays, e.g., PDH, in cellular homogenates: CON, 0.93 +/- 0.02 nmol/mg/min; NALD, 0.55 +/- 0.02; RCDP, 0.44 +/- 0.02; MIT, 0.53 +/- 0.03; PDH deficiency, 0.19 +/- 0.02.
The ability of polyamines and other cationic compounds including monoamines, amino acids, poly-L-arginine, poly-D-lysine and poly-L-lysine, to alter pyruvate dehydrogenase (PDH) activity in mitochondria from rat epididymal adipocytes was determined. PDH was assayed with the substrate [1-14C] pyruvate in the presence of 0.05 mM Ca2+ and Mg2+. Nine of the fourteen compounds tested at 0.1 mM caused a significant increase (procaine, 3-(beta-morpholinopropionyl) benzo [b]thiophene [VII], spermine, spermidine, putrescine, lysine and tryptophan) or decrease (poly-L-arginine, 3-(beta-piperidinopropionyl) benzo[b]thiophene) in PDH activity. None of these compounds nonenzymatically decarboxylated [1-14C] pyruvate to release 14CO2. NaF, a PDH phosphatase inhibitor, suppressed the stimulatory effects of those compounds tested: procaine, tryptophan, VII, spermine and spermidine. These results imply that these five compounds activate PDH activity through stimulation of the PDH phosphatase. When the Mg2+ concentration was increased from 0.05 to 4.5 mM, the stimulatory effect of spermine was increased, consistent with the finding by others that spermine lowers the Km of the enzyme for Mg2+. However, at Mg2+ concentrations greater than 0.3 mM, the stimulatory effect of VII was unaltered, procaine failed to alter PDH activity, lysine inhibited PDH activity, and poly-L-lysine stimulated PDH activity. Therefore, polyamines and other positively charged small molecules may be physiologic regulators of PDH activity.
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