TRANSPORT OF LACTATE AND OTHER MONOCARBOXYLATES ACROSS MAMMALIAN PLASMA-MEMBRANES

Transport of L-lactate across the plasma membrane is of considerable i mportance to almost all mammalian cells. In most cells a specific H+-m onocarboxylate cotransporter is largely responsible for this process; the capacity of this carrier is usually very high, to support the high rates of production or utilization of L-lactate. The best characteriz ed H+-monocarboxylate transporter is that of the erythrocyte membrane, which transports L-lactate and a wide range of other aliphatic monoca rboxylates, including pyruvate and the ketone bodies acetoacetate and beta-hydroxybutyrate. This carrier is inhibited by alpha-cyanocinnamat e derivatives and some stilbene disulfonates and has been identified a s a protein of 35-50 kDa on the basis of purification and specific lab eling experiments. Other cells possess similar alpha-cyanocinnamate-se nsitive H+-linked monocarboxylate transporters, but in some cases ther e are significant differences in the properties of these systems, suff icient to suggest the existence of a family of such carriers. In parti cular, cardiac muscle and tumor cells have transporters that differ in their K(m) values for certain substrates (including stereoselectivity for L- over D-lactate) and in their sensitivity to inhibitors. Mitoch ondria, bacteria, and yeast also possess H+-monocarboxylate transporte rs that share some properties in common with those in the mammalian pl asma membrane but are adapted to their specific roles. However, there are distinct Na+-monocarboxylate cotransporters on the luminal surface of intestinal and kidney epithelia, which enable active uptake of lac tate, pyruvate, and ketone bodies in these tissues. This article revie ws the properties of these transport systems and their role in mammali an metabolism.