Ogawa H, Gomi T, Fujioka M
Department of Biochemistry, Faculty of Medicine, Toyama Medical and Pharmaceutical University, Japan.
Int J Biochem Cell Biol. 2000 Mar;32(3):289-301. doi: 10.1016/s1357-2725(99)00113-2.
Serine hydroxymethyltransferase, a pyridoxal phosphate-dependent enzyme, catalyses the interconversion of serine and glycine, both of which are major sources of one-carbon units necessary for the synthesis of purine, thymidylate, methionine, and so on. Threonine aldolase catalyzes the pyridoxal phosphate-dependent, reversible reaction between threonine and acetaldehyde plus glycine. No extensive studies have been carried out on threonine aldolase in animal tissues, and it has long been believed that serine hydroxymethyltransferase and threonine aldolase are the same, i.e. one entity. This is based on the finding that rabbit liver serine hydroxymethyltransferase possesses some threonine aldolase activity. Recently, however, many kinds of threonine aldolase and corresponding genes were isolated from micro-organisms, and these enzymes were shown to be distinct from serine hydroxymethyltransferase. The experiments with isolated hepatocytes and cell-free extracts from various animals revealed that threonine is degraded mainly through the pathway initiated by threonine 3-dehydrogenase, and there is little or no contribution by threonine aldolase. Thus, although serine hydroxymethyltransferase from some mammalian livers exhibits a low threonine aldolase activity, the two enzymes are distinct from each other and mammals lack the "genuine" threonine aldolase.
丝氨酸羟甲基转移酶是一种依赖磷酸吡哆醛的酶，催化丝氨酸和甘氨酸的相互转化，这两种氨基酸都是合成嘌呤、胸苷酸、蛋氨酸等所需一碳单位的主要来源。苏氨酸醛缩酶催化苏氨酸与乙醛和甘氨酸之间依赖磷酸吡哆醛的可逆反应。在动物组织中尚未对苏氨酸醛缩酶进行广泛研究，长期以来人们一直认为丝氨酸羟甲基转移酶和苏氨酸醛缩酶是同一种物质，即一个实体。这是基于兔肝丝氨酸羟甲基转移酶具有一些苏氨酸醛缩酶活性这一发现。然而，最近从微生物中分离出了多种苏氨酸醛缩酶及其相应基因，并且这些酶被证明与丝氨酸羟甲基转移酶不同。对各种动物分离的肝细胞和无细胞提取物进行的实验表明，苏氨酸主要通过由苏氨酸3 - 脱氢酶启动的途径降解，苏氨酸醛缩酶的作用很小或没有作用。因此，尽管一些哺乳动物肝脏中的丝氨酸羟甲基转移酶表现出较低的苏氨酸醛缩酶活性，但这两种酶彼此不同，并且哺乳动物缺乏“真正的”苏氨酸醛缩酶。
