Amino acid catabolism by ribbed mussel (Modiolus demissus) gill tissue: studies on isolated mitochondria and the L-amino acid oxidase
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Gill tissue from Modiolus demissus has a general L-amino acid oxidase (EC 188.8.131.52) associated with proteinaceous particles sedimenting at 15,000xg. The oxidase is most active between pH 4.5 and pH 5 in citrate buffer with L-(alpha)-amino acids having three or more carbons, with no hydroxyl or methyl substitutions for hydrogens on the (beta)-carbons, and no charged groups on the (gamma)-carbon. The apparent K(,m)s for L-leucine and L-ornithine were identical (2.5 mM). Glycine, taurine, L-proline, aminooxyacetic acid, L-cycloserine, and EDTA would not act as substrates or inhibitors;Preliminary experiments with 2,4-dinitrophenol (DNP), 2,4,6-trinitrophenol (TNP), and sodium azide indicated that most of the oxygen consumption by ribbed mussel gill tissue was the result of mitochondrial respiration. A procedure utilizing isoosmotic sucrose, EGTA, defatted serum albumin, and HEPES as the isolation medium was devised for the preparation of fully coupled ribbed mussel gill mitochondria. Optimal rates of respiration and respiratory coupling required substrate, ADP, inorganic phosphate and a fairly high KC1 concentration (90 mM) in the assay medium. Preparation of gill mitochondria in isoosmotic solutions containing high KC1 concentrations (150 mM) yielded mitochondria with state 2 respiration, partially uncoupled ATP synthesis during state 3 respiration and no state 4 respiration;The presence of arginase, ornithine aminotransferase, P-5-C reductase, and proline oxidase was demonstrated in gill tissue from the ribbed mussel, Modiolus demissus. Ornithine aminotransferase and proline oxidase were found in mitochondrial fractions, and indirect evidence is presented for a mitochondrial P-5-C dehydrogenase. The proline oxidase lost its rotenone sensitivity in mechanically disrupted metochondria but retained sensitivity to antimycin A. The apparent K(,m)'s for partially purified arginase and ornithine aminotransferase were 7 mM for arginine and 4.8 mM and 2 mM for ornithine and 2-oxoglutarate, respectively. Amino acid analysis and radiotracer experiments indicated that at low concentrations proline is catabolized primarily to glutamate, organic acids and CO(,2).