The cytosolic (cAAT) and mitochondrial (mAAT) aspartate aminotransferases were partially purified from gill tissue of the ribbed mussel, Modiolus demissus, and individually characterized with respect to heat stability, electrophoretic mobility, pH optimum, K(,m) for substrates, and reactivity with aminooxyacetic acid (AOA). The mAAT was a single isozymic form with a broad pH optimum and relatively low K(,m)s for aspartate, oxaloacetate, and (alpha)-ketoglutarate and high K(,m) for glutamate. The cAAT used for the kinetic studies was a single (homozygous) isozymic form, showed great variation in K(,m) and V(,max) with pH, and high K(,m)s for the amino acid substrates and low K(,m)s for the keto acid substrates. Both enzymes were inhibited to the same degree by AOA (I(,50) = 3 x 10('-6)M). In a physiological sense, it is hypothesized that the cAAT is involved more with aspartate synthesis and the mAAT with aspartate catabolism;Differential centrifugation of ribbed mussel gill tissue homogenates and extraction of the mitochondrial fraction demonstrated that most (72%) alanine aminotransferase (AlAT) activity was mitochondrial. Subsequent characterization of the cytosolic activity indicated that it had properties identical to those demonstrated by the mitochondrial enzyme. Both enzyme fractions showed little variation in V(,max) with pH, had low K(,m)s for keto acid substrates, and were inhibited by aminooxyacetic acid (AOA), L-cycloserine, and (beta)-chloro-L-alanine. It appeared that the AlAT in ribbed mussel gill tissue was strictly mitochondrial and that alanine production and synthesis during hypoxia or during hypo- or hyperosmotic stress must be mitochondrial;The pyruvate dehydrogenase complex has been demonstrated in high speed (150,000 g) pellet preparations from sonicated ribbed mussel gill mitochondria. The complex is inhibited by low (<100 mM) chloride concentrations, succinate and ATP. ATP inhibition was enhanced by NaF and reversed by high Mg('++) concentrations in the absence of NaF. Pyruvate and thiamine pyrophosphate inhibited the inactivation by ATP. Factors involved in the ATP inhibition and Mg('++) reversal are lost with freezing or cold storage. The activity of the complex may be regulated by a phosphorylation/dephosphorylation mechanism.