Poore R E, Hurst C H, Assimos D G, Holmes R P
Department of Urology, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157, USA.
Am J Physiol. 1997 Jan;272(1 Pt 1):C289-94. doi: 10.1152/ajpcell.1997.272.1.C289.
Important features of hepatic oxalate synthesis remain uncertain despite its clinical significance. To clarify the terminal steps of the biosynthetic pathway and their modulation, we have examined oxalate and glyoxylate synthesis in vitro using isolated guinea pig peroxisomes and purified lactate dehydrogenase (LDH). Glycolate was rapidly oxidized to glyoxylate by isolated peroxisomes followed by a slower conversion of glyoxylate to oxalate. The glycolate oxidase (GO)-catalyzed conversion of glyoxylate to oxalate was strongly inhibited by physiological concentrations of glycolate and lactate. In contrast, the LDH-catalyzed conversion of glyoxylate to oxalate was only marginally affected by physiological concentrations of lactate and unaffected by physiological glycolate concentrations. This inhibition pattern suggests that LDH, not GO, catalyzes this conversion in vivo. Alanine inhibited oxalate synthesis by converting the bulk of the glyoxylate to glycine. On exposure to high alanine concentrations, however, inhibition was not complete and peroxisomes were able to convert sufficient glycolate to oxalate to account for daily endogenous oxalate production. NADH was a potent inhibitor of oxalate production by LDH by increasing glycolate formation from glyoxylate. Glycine was an ineffective source of glyoxylate, and an alkaline pH, a high-glycine concentration, and a prolonged incubation time were required to obtain a detectable synthesis. These results suggest that oxalate synthesis will be modulated by the metabolic state of the liver and resultant changes in NADH, lactate, and alanine levels.
尽管肝脏草酸合成具有临床意义，但其重要特征仍不明确。为了阐明生物合成途径的终末步骤及其调节机制，我们使用分离的豚鼠过氧化物酶体和纯化的乳酸脱氢酶（LDH）在体外研究了草酸和乙醛酸的合成。乙醇酸被分离的过氧化物酶体迅速氧化为乙醛酸，随后乙醛酸向草酸的转化较慢。乙醇酸氧化酶（GO）催化的乙醛酸向草酸的转化受到生理浓度的乙醇酸和乳酸的强烈抑制。相比之下，LDH催化的乙醛酸向草酸的转化仅受到生理浓度乳酸轻微影响，且不受生理浓度乙醇酸影响。这种抑制模式表明，在体内催化这种转化的是LDH而非GO。丙氨酸通过将大部分乙醛酸转化为甘氨酸来抑制草酸合成。然而，在高丙氨酸浓度下，抑制并不完全，过氧化物酶体能够将足够的乙醇酸转化为草酸，以解释每日内源性草酸的产生。NADH通过增加乙醛酸生成乙醇酸，是LDH产生草酸的有效抑制剂。甘氨酸不是乙醛酸的有效来源，需要碱性pH、高甘氨酸浓度和延长孵育时间才能获得可检测的合成。这些结果表明，草酸合成将受到肝脏代谢状态以及由此导致的NADH、乳酸和丙氨酸水平变化的调节。
