748 Creatine Kinase and Creatine Phosphate in Macrophages

Creatine phosphate serves as a reuseable high energy phosphate reservoir in skeletal and cardiac muscle and in the brain. These stores of creatine phosphate are thought to rephosphorylate the ATP hydrolyzed during cellular work (see 1 and 2 for review). Studies in our laboratory (3) have shown that mononuclear phagocytes obtained from the peritoneal cavities of mice also contain large stores of creatine phosphate. In resident macrophages there are 3 mol of creatine phosphate per mol of ATP. During phagocytosis, these creatine phosphate stores are consumed; presumably they are used to rephosphorylate ADP, thereby maintaining the energy charge and a constant ATP supply. The reversible transfer of the phosphoryl group of creatine phosphate to ADP is catalyzed by creatine kinase (CK), ~ an enzyme that exists in at least five isozymic forms. All forms of CK are dimers, containing subunits of ~41,000 daltons. In 1965, Dawson et al. (4) showed that these subunits can be separated into the M and B forms. The BB form is found in brain, the MM form is found in skeletal muscle, and the heterodimer MB form is found in cardiac muscle. In addition to these isozymes, Jacobs et ai. (5) demonstrated that mitochondria obtained from rat heart and skeletal muscle express a form of CK (mitochondrial CK) whose electrophoretic migration differs from that of the three known cytoplasmic forms. Subsequent reports (6, 7) have established that mitochondria have at least two unique forms of CK. In this paper, we demonstrate that the expression of CK in mouse and human mononuclear phagocytes is a developmentally regulated process occurring during the in vivo and in vitro differentiation of monocytes into macrophages. The induction of CK during in vitro cultivation occurs independently of the concentration of creatine in the medium. However, the size of the creatine phosphate pool varies directly with the concentration of creatine in the medium. The isozyme profile of CK in human macrophages differs from that observed in mouse macrophages. Human macrophages express the brain and probably the This work was supported by grant PHS NS18793 from the National Institutes of Health. Address correspondence to J. D. L. at the Department of Cellular Physiology and Immunology, The Rockefeller University, 1230 York Avenue, New York, NY 10021. Abbreviations used in this paper: CK, creatine kinase; PBS, Dulbecco's phosphate-buffered saline.