Solomon L R, Crouch J Y
Department of Medicine, Veterans Administration Medical Center, West Haven, Connecticut.
J Lab Clin Med. 1990 Aug;116(2):228-36.
Because ethanol ingestion lowers delta-aminolevulinic acid dehydratase (ALAD) activity in liver and red cells, effects of ethanol and acetaldehyde on ALAD in rat liver cytosol were studied. When added to the assay mix, as little as 0.5 mmol/L acetaldehyde competitively inhibited ALAD even in the presence of dithiothreitol, a sulfhydryl reagent. ALAD activity also fell when undiluted cytosol was incubated at 37 degrees with as little as 0.25 mmol/L acetaldehyde for 8 hours before enzyme assay. Inactivation of ALAD by acetaldehyde was prevented by the metabolic inhibitor NaF but not by the aldehyde dehydrogenase inhibitor cyanamide. Incubation of undiluted cytosol with 20 mmol/L ethanol also decreased ALAD activity, but addition of ethanol to the assay mix had no effect. Ethanol-mediated inactivation of ALAD was reduced by inhibition of alcohol dehydrogenase with 4-methylpyrazole, but ALAD activity was not decreased by incubation of undiluted cytosol with acetate or sorbitol or by addition of acetate to the assay mix. The aldehydic B6 vitamers, pyridoxal and pyridoxal phosphate, also inhibited ALAD activity when added to the assay mix. However, these vitamers increased ALAD activity and decreased acetaldehyde-mediated inactivation of ALAD when incubated for 8 hours with undiluted cytosol. We conclude that (1) acetaldehyde decreases ALAD activity both by competitive inhibition with substrate and by inactivation of enzyme protein and that (2) inactivation of ALAD by acetaldehyde may require nonoxidative metabolism of acetaldehyde. The net pharmacologic effect of B6 vitamers on ALAD activity and on inactivation of ALAD by acetaldehyde remains to be determined.
由于摄入乙醇会降低肝脏和红细胞中δ-氨基乙酰丙酸脱水酶（ALAD）的活性，因此研究了乙醇和乙醛对大鼠肝细胞溶胶中ALAD的影响。当添加到测定混合物中时，即使在存在巯基试剂二硫苏糖醇的情况下，低至0.5 mmol/L的乙醛也会竞争性抑制ALAD。在酶测定前，将未稀释的细胞溶胶在37℃下与低至0.25 mmol/L的乙醛孵育8小时，ALAD活性也会下降。乙醛对ALAD的失活作用可被代谢抑制剂NaF阻止，但不能被乙醛脱氢酶抑制剂氨甲环酸阻止。将未稀释的细胞溶胶与20 mmol/L乙醇孵育也会降低ALAD活性，但向测定混合物中添加乙醇则没有影响。用4-甲基吡唑抑制乙醇脱氢酶可减少乙醇介导的ALAD失活，但将未稀释的细胞溶胶与乙酸盐或山梨醇孵育或向测定混合物中添加乙酸盐不会降低ALAD活性。醛型维生素B6、吡哆醛和磷酸吡哆醛添加到测定混合物中时也会抑制ALAD活性。然而，当与未稀释的细胞溶胶孵育8小时时，这些维生素会增加ALAD活性并减少乙醛介导的ALAD失活。我们得出结论：（1）乙醛通过与底物的竞争性抑制和酶蛋白的失活来降低ALAD活性；（2）乙醛对ALAD的失活可能需要乙醛的非氧化代谢。维生素B6对ALAD活性和乙醛对ALAD失活的净药理作用仍有待确定。
