Martinez M, Cuskelly G J, Williamson J, Toth J P, Gregory J F
Food Science and Human Nutrition Department, University of Florida, Gainesville, FL 32611-0370, USA.
J Nutr. 2000 May;130(5):1115-23. doi: 10.1093/jn/130.5.1115.
Vitamin B-6 deficiency causes mild elevation in plasma homocysteine, but the mechanism has not been clearly established. Serine is a substrate in one-carbon metabolism and in the transsulfuration pathway of homocysteine catabolism, and pyridoxal phosphate (PLP) plays a key role as coenzyme for serine hydroxymethyltransferase (SHMT) and enzymes of transsulfuration. In this study we used [(2)H(3)]serine as a primary tracer to examine the remethylation pathway in adequately nourished and vitamin B-6-deficient rats [7 and 0.1 mg pyridoxine (PN)/kg diet]. [(2)H(3)]Leucine and [1-(13)C]methionine were also used to examine turnover of protein and methionine pools, respectively. All tracers were injected intraperitoneally as a bolus dose, and then rats were killed (n = 4/time point) after 30, 60 and 120 min. Rats fed the low-PN diet had significantly lower growth and plasma and liver PLP concentrations, reduced liver SHMT activity, greater plasma and liver total homocysteine concentration, and reduced liver S-adenosylmethionine concentration. Hepatic and whole body protein turnover were reduced in vitamin B-6-deficient rats as evidenced by greater isotopic enrichment of [(2)H(3)]leucine. Hepatic [(2)H(2)]methionine production from [(2)H(3)]serine via cytosolic SHMT and the remethylation pathway was reduced by 80.6% in vitamin B-6 deficiency. The deficiency did not significantly reduce hepatic cystathionine-beta-synthase activity, and in vivo hepatic transsulfuration flux shown by production of [(2)H(3)]cysteine from the [(2)H(3)]serine increased over twofold. In contrast, plasma appearance of [(2)H(3)]cysteine was decreased by 89% in vitamin B-6 deficiency. The rate of hepatic homocysteine production shown by the ratio of [1-(13)C]homocysteine/[1-(13)C]methionine areas under enrichment vs. time curves was not affected by vitamin B-6 deficiency. Overall, these results indicate that vitamin B-6 deficiency substantially affects one-carbon metabolism by impairing both methyl group production for homocysteine remethylation and flux through whole-body transsulfuration.
维生素B-6缺乏会导致血浆同型半胱氨酸轻度升高，但其机制尚未明确。丝氨酸是一碳代谢以及同型半胱氨酸分解代谢转硫途径中的一种底物，而磷酸吡哆醛（PLP）作为丝氨酸羟甲基转移酶（SHMT）和转硫途径中各酶的辅酶发挥关键作用。在本研究中，我们使用[(2)H(3)]丝氨酸作为主要示踪剂，以研究营养充足和维生素B-6缺乏的大鼠[分别给予7和0.1毫克吡哆醇（PN）/千克饮食]体内的再甲基化途径。[(2)H(3)]亮氨酸和[1-(13)C]蛋氨酸也分别用于研究蛋白质和蛋氨酸池的周转率。所有示踪剂均以大剂量腹腔注射，然后在30、60和120分钟后处死大鼠（每个时间点n = 4）。喂食低PN饮食的大鼠生长显著减缓，血浆和肝脏PLP浓度降低，肝脏SHMT活性降低，血浆和肝脏总同型半胱氨酸浓度升高，肝脏S-腺苷甲硫氨酸浓度降低。[(2)H(3)]亮氨酸的同位素富集程度更高，这表明维生素B-6缺乏的大鼠肝脏和全身蛋白质周转率降低。在维生素B-6缺乏的情况下，通过胞质SHMT和再甲基化途径由[(2)H(3)]丝氨酸生成肝脏[(2)H(2)]蛋氨酸的量减少了80.6%。这种缺乏并未显著降低肝脏胱硫醚-β-合酶的活性，并且由[(2)H(3)]丝氨酸生成[(2)H(3)]半胱氨酸所显示的体内肝脏转硫通量增加了两倍多。相比之下，在维生素B-6缺乏的情况下，[(2)H(3)]半胱氨酸在血浆中的出现量减少了89%。由[1-(13)C]同型半胱氨酸/[1-(13)C]蛋氨酸富集程度与时间曲线下面积之比所显示的肝脏同型半胱氨酸生成速率不受维生素B-6缺乏的影响。总体而言，这些结果表明，维生素B-6缺乏通过损害同型半胱氨酸再甲基化的甲基生成以及全身转硫通量，对一碳代谢产生了实质性影响。
