Uptake of biotin by native Xenopus laevis oocytes

Abstract

The present study examined biotin uptake by Xenopus laevis oocytes in vitro. Uptake of low (0.03 microM) and high (10 microM) concentrations of biotin was linear with time for up to 4 h of incubation and occurred with little initial binding to oocytes. Uptake of biotin was dependent on extracellular Na+ concentration [Na+]o and was severely inhibited when Na+ was replaced by other monovalent cations [choline, tetraethylammonia, Li+, and tris(hydroxymethyl)aminomethane]. The initial rate of biotin uptake was saturable as a function of concentration with an apparent Michaelis constant of 3.9 +/- 0.5 microM and maximum velocity of 1,559 +/- 70 fmol.oocyte-1.h-1. Addition to the incubation medium of biotin structural analogues desthiobiotin and thioctic acid caused significant and concentration-dependent inhibition in the uptake of [3H]biotin. This inhibition was found to be competitive in nature with inhibition constant values of 9 and 17.5 microM. In contrast, neither the structural analogue biocytin nor biotin methyl ester (compounds in which the carboxyl group of the valeric acid moiety is blocked) showed any effect on the uptake of [3H]biotin. Biotin uptake was significantly blocked by the metabolic inhibitors dinitrophenol, cyanide, and azide and by incubation at 4 degrees C. Also, the sulfhydryl group blocker p-(chloromercuri)phenylsulfonate caused significant inhibition in biotin uptake. These results demonstrate that Xenopus oocytes possess an uptake system for biotin in its cell membrane that is Na+, energy, and temperature dependent. These characteristics of biotin uptake are similar to those reported in mammalian cells. It is suggested that Xenopus oocytes might be a useful in vitro model system to study the details of the mechanisms and regulation of biotin movement across biological membranes.(ABSTRACT TRUNCATED AT 250 WORDS)