Uptake and processing of serine: pyruvate aminotransferase precursor by rat liver mitochondria in vitro and in vivo.
Processing and uptake of the precursor of serine: pyruvate aminotransferase [EC 2.6.1.51] by mitochondria were studied in vitro and in vivo. Serine: pyruvate aminotransferase was synthesized mainly on free ribosomes as judged by immunoprecipitation of puromycin-labeled nascent peptides prepared from free and bound ribosomes. The precursor of rat liver serine:pyruvate aminotransferase (pSPT) synthesized in vitro was post-translationally processed to an apparently mature form by isolated rat liver mitochondria. Available evidence indicated that the processed product was localized in the matrix of mitochondria. Mature serine:pyruvate aminotransferase did not inhibit the in vitro processing, suggesting that the extra peptide was necessary for the mitochondrial uptake of the precursor. In the livers of rats fed a vitamin B6-deficient high-protein diet, the induction by glucagon of serine:pyruvate aminotransferase occurred and most of the induced enzyme existed in mitochondria as the apo-form, suggesting that pSPT was taken up by mitochondria and processed in the apo-form under the conditions employed. In the in vitro system, on the other hand, the processing of pSPT proceeded both in the absence and presence of pyridoxal 5'-phosphate. Should the precursor also bind the prosthetic molecule, therefore, it would be transported into mitochondria in both the apo- and holo-forms. When isolated rat hepatocytes were labeled with [35S]methionine, labeled pSPT appeared in the cytosolic fraction and was transported rapidly into mitochondria in association with the processing. This uptake and processing were inhibited by a fluorescent laser dye, rhodamine 123, and the precursor accumulated in the cytosol in the presence of the dye.
D000818:Animals; D001692:Biological Transport; D004792:Enzyme Precursors / Q000378:metabolism*; D007120:Immunochemistry; D066298:In Vitro Techniques; D008297:Male; D008930:Mitochondria, Liver / Q000201:enzymology*; D011499:Protein Processing, Post-Translational; D011736:Pyridoxine / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains; D013347:Subcellular Fractions / Q000201:enzymology; D000637:Transaminases / Q000378:metabolism*
D004792:Enzyme Precursors; D000637:Transaminases; C022638:serine-pyruvate aminotransferase; D011736:Pyridoxine