Respiratory and enzymatic properties of squid heart mitochondria

Mitochondria isolated from systemic hearts of the squid Illex illecebrosus showed high respiratory control ratios, and, with appropriate substrates, the expected ADP/O ratios. Of amino acids tested, proline and ornithine were oxidized at highest rates; of carboxylates, malate, succinate and pyruvate gave the highest state-3 respiration rates. Pyruvate oxidation is enhanced with proline, ornithine, and 1-pyrroline-5-carboxylate (pyrroline carboxylate) all of which can serve to augment the Krebs cycle. However, proline, ornithine and pyrroline carboxylate oxidation is not similarly dependent upon pyruvate. Rotenone inhibited state-3 respiration of malate, proline, ornithine and pyrroline carboxylate. Neither intermediates of fatty acid oxidation nor glycerol 3-phosphate were utilized at significant rates. Key enzymes in proline and ornithine oxidation, i.e. proline dehydrogenase, pyrroline-carboxylate dehydrogenase, ornithine aminotransferase, and glutamate dehydrogenase were located in the mitochondria. The synthesis of proline is catalyzed by pyrroline-carboxylate reductase, which was found exclusively in the cytosol. The respiration, phosphorylation and enzyme data taken together suggest that the main carbon sources for heart mitochondria of Illex are pyruvate plus the proline and ornithine pool.