Studies have been directed to the determination of the structure and mechanism of action of adenylate kinases. A related enzyme, GTP-AMP phosphotransferase from beef heart mitochondria has been purified. This enzyme is of special interest because unlike myokinase the two nucleotide substrates have different base moieties so that added substrate is known to bind at the triphosphate or the monophosphate binding site. Peptides for amino acid sequencing are being prepared and crystals for X-ray work are being grown. Intrinsic fluorescence studies on the GTP-AMP phosphotransferase and on Baker's yeast adenylate kinase are being completed. Chemical studies of the active site groups are continuing. Other adenylate kinases under study include that from Paracoccus denitrificans.