Dale R A
Biochim Biophys Acta. 1978 Dec 18;544(3):496-503. doi: 10.1016/0304-4165(78)90324-0.
There is doubt about the L-threonine 3-dehydrogenase (EC 1.1.1.103) and threonine aldolase (EC 2.1.2.1) catabolic pathways of L-threonine in mammals which are believed to produce aminoacetone and glycine plus acetaldehyde, respectively. L-Threonine 3-dehydrogenase in disrupted guinea-pig liver mitochondria was investigated in a reaction mixture containing L-threonine without and with CoA and oxaloacetate; L-[U-14C]threonine was included in four similar experiments for autoradiograms. Threonine aldolase was examined in similar mitochondria from liver and kidney. CoA reduced the aminoacetone formed from L-threonine to 10-14% and CoA plus oxaloacetate produced citrate (from CoASAc) in approximately equal amounts to the decrease in aminoacetone. Autoradiograms confirmed the decrease in aminoacetone with the simultaneous appearance of citrate and glycine. No evidence was obtained that threonine aldolase catabolised L-threonine at the concentration used to assay the dehydrogenase. It is concluded that 2-amino-3-oxobutyrate (precursor of aminoacetone), which is produced from L-threonine by L-threonine 3-dehydrogenase, undergoes CoA-dependent cleavage to glycine and CoASAc by 2-amino-3-oxobutyrate-CoA ligase. The results suggest that the coupling of these enzymes provides a new pathway for the catabolism of threonine in mammals.
关于哺乳动物中L-苏氨酸的L-苏氨酸3-脱氢酶（EC 1.1.1.103）和苏氨酸醛缩酶（EC 2.1.2.1）分解代谢途径存在疑问，据信这两条途径分别产生氨基丙酮、甘氨酸和乙醛。在含有L-苏氨酸且分别添加和不添加辅酶A及草酰乙酸的反应混合物中，研究了豚鼠肝脏破碎线粒体中的L-苏氨酸3-脱氢酶；在四个类似实验中加入L-[U-¹⁴C]苏氨酸用于放射自显影片。在来自肝脏和肾脏的类似线粒体中检测了苏氨酸醛缩酶。辅酶A将由L-苏氨酸形成的氨基丙酮减少至10 - 14%，辅酶A加草酰乙酸产生的柠檬酸（来自辅酶A - 乙酰基）量与氨基丙酮的减少量大致相等。放射自显影片证实了氨基丙酮的减少以及同时出现的柠檬酸和甘氨酸。未获得证据表明在所用于检测脱氢酶的浓度下，苏氨酸醛缩酶分解代谢L-苏氨酸。结论是，由L-苏氨酸3-脱氢酶从L-苏氨酸产生的2-氨基-3-氧代丁酸（氨基丙酮的前体），通过2-氨基-3-氧代丁酸-辅酶A连接酶进行依赖辅酶A的裂解生成甘氨酸和辅酶A - 乙酰基。结果表明这些酶的偶联为哺乳动物中苏氨酸的分解代谢提供了一条新途径。
