Role of high affinity folate binding protein on the plasma distribution of tetrahydrofolate in pigs. Sasaki, Kazuaki, Masahiro Natsuhori, Minoru Shimoda, Yukiko Saima, and Ei-Ichi Kokue. Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183, Japan, *Present address: Advanced Research Laboratory, Hitachi Co., Ltd., Hatoyama, Saitama, 350-03 Japan.
APStracts 2:0215R, 1995.
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 THF existed as a bound form, whereas THF both in plasma ultrafiltrate and in plasma ultrafiltrate plus porcine albumin was degraded rapidly and disappeared soon after addition of THF. These results suggested 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 nM and 70 nM, respectively, indicating that more than 98 % of endogenous plasma THF existed as bound form with HFBP. Porcine albumin was not essentially a protein which 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. 

Received 13 December 1994; accepted in final form 6 July 1995.
APS Manuscript Number R706-4.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.