Mitochondrial neutral amino acid transport: evidence for a carrier mediated mechanism

Abstract

Swelling of rat liver mitochondria induced by various neutral amino acids indicates that the L isomers of serine, alanine, methionine, valine, threonine and leucine enter the mitochondrial matrix by a stereospecific process. Chemical modification of mitochondria with diazobenzenesulfonate or p-mercuribenzoate inhibited the rapid uptake of these compounds, as well as that of L-proline and glycine, to various extents. Diazobenzenesulfonate did not inhibit the transport of compounds that enter the mitochondrial matrix by simple diffusion, i.e., thiocyanate, nitrate, formate, bicarbonate and acetate. Inhibition by p-mercuribenzoate was reversed by treatment with dithiothreitol. Inclusion of various neutral amino acids in the p-mercuribenzoate preincubation mixture substantially prevented inactivation of transport. Glycine, D- or L-serine, L-threonine, L-methionine, L-alanine and L-valine all individually protected against the inactivation of glycine, L-serine, L-threonine, L-methionine, L-alanine, L-valine, L-proline, and .beta.-alanine transport, while L-proline, .beta.-alanine, L-leucine, L-isoleucine and L-histidine did not protect. L-Arginine potentiated p-mercuribenzoate inactivation of neutral amino acid transport. These findings are consistent with the presence of an inner membrane neutral amino acid carrier system having broad substrate specificity, although the presence of multiple carriers cannot be conclusively eliminated.