Modified phosphoribosylpyrophosphate (PRPP) radioenzymatic assay: increased sensitivity, technical simplification and new applications.
Phosphoribosylpyrophosphate in amounts as low as 25 pmol could be reliably and economically measured with a CO2-releasing radioenzymatic assay when appropriate technical modifications were introduced. The concentration of commercially available phosphoribosylpyrophosphate used for reference standards was ascertained by a method based on the utilization of phosphoribosylpyrophosphate by hypoxanthine catalyzed by hypoxanthine phosphoribosyltransferase from red blood cell lysates. The addition of inorganic phosphate increased intracellular phosphoribosylpyrophosphate levels in HL-60 cell lysates and can be used to amplify low levels of phosphoribosylpyrophosphate. This phosphoribosylpyrophosphate assay amplified by inorganic phosphate has been developed to assay perturbations in the purine biosynthetic nucleotide pathway in response to various chemotherapeutic agents, such as anti-folates, or as a result of folate deficiency.