DOES HEPATIC ATP DEPLETION IMPAIR GLYCINE CONJUGATION IN-VIVO

Conjugation with glycine, a reaction important in the elimination of c arboxylic acids (e.g. benzoic and salicylic acids), takes place in hep atic mitochondria and uses ATP, coenzyme A, and glycine. Although norm al ATP supply does not limit glycine conjugation in vivo (Gregus, Z., et al., Drug Metab. Dispos. 20, 234, 1992), ATP deficiency may impair it. This hypothesis was tested by examining the effect of ATP depletor s (oligomycin, fructose, and ethionine) on glycine conjugation and eli mination of benzoic acid in rats. Pretreatment with the mitochondrial ATP synthesis inhibitor oligomycin (0.5-2 mg/kg, intraportally) decrea sed glycine conjugation of benzoic acid markedly and in a dose-depende nt manner, as indicated by the delayed elimination of benzoate and del ayed appearance of benzoylglycine in blood. Oligomycin also dramatical ly diminished urinary excretion of benzoylglycine, because it inhibite d not only formation of benzoylglycine from benzoate, but also the ren al transport of benzoylglycine. Treatment with fructose (a consumer of both cytosolic and mitochondrial ATP) or ethionine (a consumer of cyt osolic ATP) depleted hepatic ATP from similar to 2.5 mu mol/g to level s comparable with those observed after administration of 1 mg/kg oligo mycin (similar to 1.2 mu mol/g). Despite this, elimination of benzoate and formation of benzoylglycine were decreased less by fructose than by oligomycin and only negligibly by ethionine. ATP depletors did not influence hepatic glycine levels, and only oligomycin lowered coenzyme A levels in liver. However, the oligomycin-induced decline of hepatic coenzyme A levels was delayed, contrary to impairment of glycine conj ugation, which was almost immediate. In summary, impairment of benzoyl glycine formation by ATP depletors apparently correlates with their ca pacity to diminish ATP levels in hepatic mitochondria (i.e. at the sit e of glycine conjugation). These observations suggest that limited ava ilability of mitochondrial (but not cytosolic) ATP reduces glycine con jugation capacity, Therefore, mitochondrium toxic agents and pathologi cal mitochondrial injuries acting in liver may compromise glycine conj ugation by decreasing ATP supply.