Utilization of biotin in proliferating human lymphocytes

Lymphocytes are part of the immune system and respond to antigenic stimulation with proliferation. We sought to determine whether mitogen- stimulated, proliferating lymphocytes increase the cellular uptake of biotin and, if so, to identify mechanisms that mediate the increase. Lymphocytes were isolated from human peripheral blood; proliferation of lymphocytes was induced by incubation with pokeweed lectin, concanavalin A or phytohemagglutinin. Biotin uptake was quantitated by determination of [³H] uptake into the lymphocytes during incubation with [³H]biotin after establishing that [³H]biotin is not metabolized within the lymphocytes during the incubation period (<5%). Biotin uptake into proliferating lymphocytes increased to 278-722% of the control values for nonproliferating lymphocytes. Kinetic analysis of biotin transport provided evidence that the increase is mediated by an increased number of transporters on the cell surface rather than by an increase in transporter affinity. Cycloheximide, an inhibitor of protein synthesis, completely suppressed the mitogen-stimulated increase in biotin transport. This observation is consistent with the hypothesis that proliferating lymphocytes increase biotin uptake by increasing the synthesis of new transporters. Biotin affinity and structural specificity were similar in proliferating and nonproliferating lymphocytes, suggesting that mitogens induced an increase in the number of the same transporter molecule that mediates transport in unstimulated lymphocytes. Mitogen-stimulated lymphocytes exhibited 2.5 times greater activities of biotin-dependent β-methylcrotonyl-CoA carboxylase compared with time 0 (at 72 h after addition of mitogen). This observation is consistent with the hypothesis that proliferating lymphocytes increase biotin uptake at least in part to provide adequate coenzyme for biotin-dependent carboxylases.