Control of sn-glycerql 3-phosphate oxidation in brown adipose tissue mitochondria by calcium and ACYL-CoA

Abstract

1. 1. Brown adipose tissue mitochondria isolated in the absence of cation chelators possess a remarkable capacity for oxidation of sn-glycerol 3-phosphate. This can be inhibited by the addition of EDTA or ethylene glycol-bis(aminoethyl)tetraacetic acid (EGTA) to the incubation medium and subsequently reversed by the addition of an excess of Ca 2+. The levels of free calcium required to activate this process were found to vary between 1.0 · 10 -8 and 6.5 · 10 -7 M. 2. 2. The oxidation of sn-glycerol 3-phosphate exhibits allosteric kinetics, and calcium acts as a positive modifier by lowering the apparent K m for the substrate. 3. 3. In the presence of calcium, the oxidation of sn-glycerol 3-phosphate can be inhibited by very low concentrations of palmitoyl-CoA, which is a much more potent inhibitor than palmitoyl carnitine or palmitic acid. This inhibition can be prevented or reversed by albumin and is competitive with respect to sn-glycerol 3-phosphate with an apparent K i of about 2 nmoles palmitoyl-CoA/mg mitochondrial protein. 4. 4. The inhibition by palmitoyl-CoA can be mimicked by stimulating the formation of acyl-CoA derivatives from the endogenous pool of mitochondrial fatty acids by the addition of ATP. CoA, not inhibitory by itself, enhances the ATP effect which can be reversed or prevented by albumin. Such an effect is not observed in the absence of Mg 2+ or if the fatty acid-activating enzyme is inhibited by nagarse. 5. 5. Calcium and palmitoyl-CoA both act on the mitochondrial sn-glycerol 3-phosphate dehydrogenase (EC 1.1.99.5) rather than on the electron transport chain. 6. 6. The reversible nature of calcium and acyl-CoA effects, and the relatively low levels at which they act suggest that both effects might be physiologically important for the control of sn-glycerol 3-phosphate metabolism in brown adipose tissue.