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For the enzymatic conversion of 5´-XMP to 5´-GMP, partially purified XMP aminase from Escherichia coli was coupled with the yeast, Saccharomyces cerevisiae, capable of ATP regeneration through glycolytic pathway. In order to elevate the level of XMP aminase in E. coli, guaB^-(IMP dehydrogenase-less) mutant were introduced, and the yeast used as ATP supplier was treated by some method to increase its membrane permeability. The optimum conditions for efficient conversion reaction by energy-coupled system were investigated. As the results, a CH 41, guaB^- mutant of E. coli K-12, showed 2.75 fold increase in the level of XMP aminase, compared with its parent cell. And the lyophylized yeast was the most effective as the ATP supplier. The optimum temperature and pH of conversion reaction were 40℃ and pH 7.4, and the highest conversion ratio was shown under the reaction condition of 100 mM glucose, 100 mM inorganic phosphate and 6 mM AMP. When 36 units/㎖ XMP aminase used under the above conditions, the amount of 60 ㎎/㎖ yeast was sufficient to be used. Under the optimum condition, 71% of 1.8 mM(65.6 ㎎/100 ㎖) 5´-XMP was converted to 5´-GMP within 8 hr.
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