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Regulation of the glycine cleavage system in isolated rat liver mitochondria.

1983; Elsevier BV; Volume: 258; Issue: 5 Linguagem: Inglês

10.1016/s0021-9258(18)32819-9

1083-351X

R K Hampson, Lynn L. Barron, Merle S. Olson,

Neuroscience and Neuropharmacology Research

Catabolism of glycine by the glycine cleavage system was investigated in isolated rat liver mitochondria.Metabolic flux through the glycine cleavage system was monitored by measuring the production of I4CO2 from [l-'4C]glycine.Glycine decarboxylation by mitochondria maintained in State 3 was maximal as the glycine concentration approached 20 m~ and exhibited an activity of 1-2 nmol-mg"-min-l and an apparent K, of approximately 4 m ~.Glycine decarboxylation activity was highly sensitive to the mitochondrial oxidation-reduction state as glycine decarboxylation was stimulated in State 3 over State 4 and was maximal in the uncoupled state.Alternatively, respiratory inhibitors such as rotenone and reducing substrates such as succinate, a-ketoglutarate, glutamate plus tmalate, pyruvate plus L-malate, and &-hydroxybutyrate greatly inhibited glycine decarboxylation.In addition, flux through the glycine cleavage system was found to be sensitive to the oxidation-reduction state of both the NAD@) and NADP(H) couples of this liver mitochondrial system.Uncoupling liver mitochondria greatly stimulated glycine decarboxylation but caused only a small decrease in NADH and a large decrease in the NADPH levels.Propionate stimulated mitochondrial glycine decarboxylation and caused a corresponding oxidation of NADPH.Moreover, if the NAD(H) oxidation-reduction state was buffered with varying ratios of @-hydroxybutyrate and acetoacetate, glycine decarboxylation was altered in a fashion consistent with an oxidationreduction effect.Repeating this experiment in the presence of 5 m~ propionate resulted in rates of glycine decarboxylation which varied over a &fold greater range. The primary pathway for glycine catabolism in mammals(1) is via the glycine cleavage system (2).This enzyme system, located exclusively in the mitochondrial compartment, catalyzes the reversible cleavage of glycine to yield carbon dioxide, ammonia, 5N,10N-methylenetetrahydrofolate, and one reducing equivalent as NADH + H'.Four enzymatic components constitute the glycine cleavage system: a pyridoxal phosphatedependent glycine decarboxylase (31, a lipoic acid-containing aminomethyl transferase (4), a 5N,'oN-methylenetetrahydrofolate-synthesizing protein (5), and the flavoprotein, dihydrolipoyldehydrogenase (6).These four catalytic subunits, also referred to as the P, €3, T, and L proteins, respectively, likely