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This report describes sensitive, precise microtechniques that allow quantification of glucose and alanine metabolism in vivo using stable isotope tracers. By combined gas chromatography--mass spectrometry and selected ion monitoring--deuterium and carbon-13 enrichment in blood glucose and alanine were measured with an error of less than 2 per cent. Estimation of glucose and alanine flux in dogs by stable isotope tracer techniques was compared with simultaneous measurements made in the same animal with conventional radiotracer dilution methods. Application of the described stable isotope methods to determination of glucose and alanine turnover as well as alanine-2,3-13C incorporation into glucose in adult men confirmed the safety and validity of these techniques for human investigation.

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In a previous study (Candy, D. J. (1967), Biochem. J. 103, 666) a locust fat body extract was shown to convert myo-to scyZZo-inositol in a reaction involving NADT. The same enzyme preparation reduced scyllomyo-inosose (2,4,6/ 3,5-pentahydroxycyclohexanone) to .scyZZo-inositol, however, with NADPH. This difference in cofactor requirements suggested the presence of more than one enzyme and led us to the study of these systems in the American cockroach, Periplaneta americana L. Dialyzed fat body homogenates were the enzyme source. Analysis of the products of the enzyme reactions was by gas chromatography and by gas chromatography-mass spectrometry of the trimethylsilyl ethers. An inositol: NAD+ epimerase (pH optimum 7.5-8.0) was found which was capable of carrrying out the following inositol conversions: epi-> myo-[scyllo-+ chiro-+ neo-]. The system epimerizes both (d)-and (L)cZiZra-inositol but not mucoinositol. The degree to which the epimerase operates on the t From the Departments of Psychiatry and Biological Chemistry,

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