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1. The concentration of adenylate kinase in carp muscle is about 0.3 mg/g. An improved isolation procedure makes use of a dilute solution of the substrates, ATP and AMP, to elute the enzyme from a phosphocellulose column in overall yields of 60% before crystallization. By the hexokinase - pH-stat assay the specific activity is 3550 units/mg. The preparation has been found to be essentially homogeneous by dodecylsulfate gel electrophoresis, isoelectrofocusing and gel filtration. 2. The molecular weight has been determined to be 22000 by several methods. The absorbance of a 1% solution at 280 nm in 6.9 and the isoelectric point by electrofocusing is pH 5.9. 3. The crystals of carp adenylate kinase have the space group P4122 or P4322. 4. The amino acid composition has been determined. There is no tryptophan, no cysteine. There is one amino acid residue each of cysteine and histidine which are at or close to the catalytic center. Several peptides derived by tryptic hydrolysis have been isolated and identified with corresponding peptides of porcine adenylate kinase. Consideration is given to histidine and cysteine being a part of the active site. [ABSTRACT FROM AUTHOR]