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Elsewhere in this book the important role of creatine kinase and its metabolites in high energy phosphate metabolism and transport in muscle cells has been reviewed. The emphasis of this review article is mainly on the compartmentalized catalytic activity of adenylate kinase in relation to creatine kinase isoenzymes, and other enzymes of energy production and utilization processes in muscle cells. At present the role of adenylate kinase is considered simply to equilibrate the stores of adenine nucleotides. Recent studies by us and others, however, suggest an entirely new view of the metabolic importance of adenylate kinase in muscle function. This view offers a closer interaction between adenylate kinase and creatine kinase, in the process of energy production (at mitochondrial and glycolytic sites), and energy utilization (at myofibrillar sites and perhaps other sites such as sarcoplasmic reticular, sarcolemmal membrane, etc.), thus being an integral part of the high energy phosphate transport system. This review article opens up the opportunity to further examine the metabolism of adenine nucleotides and their fluxes through the adenylate kinase system in intact muscle cells. Using an intact system, having a preserved integrity of their compartmentalized enzymes and substrates, is essential in clarifying the exact role of adenylate kinase in high energy phosphate metabolism in muscle cells.

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To measure the actual percentage of intracellular free creatine participating in the process of energy transport, the incorporation of [1-'4C]creatine into the "free"

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