UPTAKE OF RIBOFLAVIN BY INTESTINAL BASOLATERAL MEMBRANE-VESICLES - A SPECIALIZED CARRIER-MEDIATED PROCESS

The mechanism of riboflavin (RF) uptake by intestinal basolateral memb rane vesicles (BLMV) was examined in this study. BLMV were isolated by an established Percoll-gradient methodology from rabbit small intesti ne. Uptake of riboflavin was mainly the result of transport of the sub strate into an osmotically active intravesicular space with less bindi ng to membrane surfaces. Uptake of RF with time was similar in the pre sence of a Na+ and a K+ gradient (out > in) and was not significantly influenced by changes in incubation buffer pH. The initial rate of upt ake of riboflavin as a function of concentration was saturable in both jejunal and ileal BLMV and occurred with apparent K(m) values of 5.0 muM and 4.4 muM and V(max) values of 91.6 and 60.8 pmol/mg protein per 5 s, respectively. Unlabeled riboflavin and the structural analogues lumiflavin, isoriboflavin and 8-aminoriboflavin all caused significant inhibition (but to different degrees) in the uptake of [H-3]riboflavi n. On the other hand, 8-hydroxyriboflavin, lumichrome, lumazine and D- ribose failed to inhibit [H-3]riboflavin uptake. Trans-stimulation of [H-3]riboflavin efflux from preloaded BLMV by unlabeled riboflavin or lumiflavin was also observed. Altering transmembrane electrical potent ial by anion substitution and valinomycin-induced K+ diffusion did not affect the riboflavin uptake process. These results demonstrate the e xistence of a specialized carrier-mediated mechanism for riboflavin up take by intestinal BLMV. Furthermore, the system appears to transport the vitamin by a process which is Na+- and pH-independent, and electro neutral in nature.