Summary
Adenine uptake into isolated platelet membranes had about the same Km (151 ± 21 •
9 nM) as uptake into intact cells (159 ± 21 nM) and was also competitively inhibited
by papaverine and hypoxanthine. No uptake occurred at 0° and accumulated adenine was
converted to AMP. AMP was not firmly bound to protein as judged by chromatography
of triton X-100 solubilized membranes on Sephadex G25. The pH optimum for adenine
uptake was at pH 5-5. Exogenous 5-phosphoribosyl-l-pyrophos- phate strongly stimulated
uptake. These data may be explained by uptake of adenine by facilitated diffusion
followed by conversion to AMP by adenine phosphoribosyltransferase but group translocation
cannot be entirely excluded.
