Sasaki K, Natsuhori M, Shimoda M, Saima Y, Kokue E
Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Japan.
Am J Physiol. 1996 Jan;270(1 Pt 2):R105-10. doi: 10.1152/ajpregu.1996.270.1.R105.
Stability and protein-binding properties of tetrahydrofolate (THF) in pig plasma were studied in vitro. THF in plasma was stable for more than 120 min when it existed in a bound form, whereas THF both in plasma ultrafiltrate and in plasma ultrafiltrate plus porcine albumin was degraded rapidly and disappeared soon after its addition. These results suggest that high-affinity folate-binding protein (HFBP) is related to the stability of THF. THF-protein binding kinetic analysis showed that porcine plasma had HFBP and low-affinity binding protein (albumin) for THF. Dissociation constant and maximal binding capacity of HFBP were calculated to be 0.4 and 70 nM, respectively, indicating that > 98% of endogenous plasma THF existed in bound form with HFBP. Porcine albumin was not essentially a protein that binds and protects endogenous THF from degradation. We conclude that most endogenous THF binds to HFBP and only the unbound form of THF is rapidly degraded in pig plasma. HFBP protects THF from degradation and allows THF to exist stably in pig plasma. In addition, HFBP may govern the species specificity of plasma folate distribution in pigs.
体外研究了猪血浆中四氢叶酸（THF）的稳定性和蛋白质结合特性。当血浆中的THF以结合形式存在时，其在120分钟以上是稳定的，而血浆超滤物中的THF以及血浆超滤物加猪白蛋白中的THF在添加后迅速降解并很快消失。这些结果表明高亲和力叶酸结合蛋白（HFBP）与THF的稳定性有关。THF-蛋白质结合动力学分析表明，猪血浆中存在THF的HFBP和低亲和力结合蛋白（白蛋白）。计算得出HFBP的解离常数和最大结合容量分别为0.4和70 nM，表明>98%的内源性血浆THF以与HFBP结合的形式存在。猪白蛋白本质上不是一种能结合并保护内源性THF不被降解的蛋白质。我们得出结论，大多数内源性THF与HFBP结合，只有未结合形式的THF在猪血浆中迅速降解。HFBP保护THF不被降解，并使THF在猪血浆中稳定存在。此外，HFBP可能决定猪血浆中叶酸分布的物种特异性。
