SUMMARY.A simple spectrophotometric assay of phosphoribosyl pyrophosphate synthetase from human erythrocytes is described. The kinetic properties of the enzyme have been determined and are in good agreement with published results. Conditions for storing haernolysates at -20°C are described. The method is suitable for semi-automation.The synthesis of phosphoribosyl pyrophosphate (PRPP) from ATP and ribose-5-phosphate (R-5-P) is catalysed by PRPP synthetase (EC.2. 7.6.1.) and the reaction involves the direct transfer of the terminal pyrophosphate of ATP to the I' carbon of R-5-P.' R-5-P + ATP PRPP synthetase PRPP + AMP )

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An adaptation of a radioimmunoassay for plasma oestriol for assaying urine oestriol is presented. The method has been shown to be precise, reliable, and directly comparable with the manual spectrophotometric technique for urine oestriol.