ROLE OF HIGH-AFFINITY FOLATE-BINDING PROTEIN IN THE PLASMA DISTRIBUTION OF TETRAHYDROFOLATE IN PIGS

Stability and protein-binding properties of tetrahydrofolate (THF) in pig plasma were studied in vitro. THF in plasma was stable for more th an 120 min when it existed in a bound form, whereas THF both in plasma ultrafiltrate and in plasma ultrafiltrate plus porcine albumin was de graded 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 tha t porcine plasma had HFBP and low-affinity binding protein (albumin) f or THF. Dissociation constant and maximal binding capacity of HFBP wer e calculated to be 0.4 and 70 nM, respectively, indicating that >98% o f endogenous plasma THF existed in bound form with HFBP. Porcine album in was not essentially a protein that binds and protects endogenous TH F from degradation. We conclude that most endogenous THF binds to HFBP and only the unbound form of THF is rapidly degraded in pig plasma. H FBP protects THF from degradation and allows THF to exist stably in pi g plasma. In addition, HFBP may govern the species specificity of plas ma folate distribution in pigs.