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Publisher Summary This chapter illustrates the expressions of citrullinemia in skin fibroblast cultures. Citrullinemia is a rare disease in man which is believed to be inherited as an autosomal recessive condition. The defect is because of a deficiency of the urea cycle enzyme argininosuccinic acid synthetase (ASA synthetase), which catalyzes the condensation of citrulline with aspartic acid to form argininosuccinic acid. This enzymatic activity has been demonstrated in human diploid fibroblast cell cultures in three ways: (a) by direct enzyme assay, (b) by the measurement of the incorporation of isotopically labeled citrulline into the acid-insoluble fraction (cell protein), and (c) by the measurement of cell multiplication when citrulline is substituted for arginine as an essential cell nutrient. Because cells with ASA synthetase deficiency as well as argininosuccinase (ASAase) deficiency (the next enzyme in the urea cycle which converts argininosuccinic acid to arginine) grow poorly when citrulline is substituted for arginine in the medium, these mutant cell lines have been of interest as auxotrophic cells in genetic experiments. The chapter describes the mechanism of colorimetric and radioisotope ASA synthetase assays. The chapter discusses the measurement of citrulline incorporation into cell protein. |
