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Heavy Metal Impurities Impair the Spectrophotometric Assay of Ribulose Bisphosphate Carboxylase Activity ¹

^a Department of Biology, Washington University, St. Louis, Missouri 63130

An inverse relationship between the concentration of ribose 5-phosphate and apparent ribulose bisphosphate carboxylase activity was observed. The Lilley-Walker assay spectrophotometric assay, in which the 3-phosphoglyceric acid-dependent oxidation of reduced pyridine nucleotide is measured, is shown to be highly sensitive to inhibition by heavy metals. Analysis of the purity of reagents showed that ribose 5-phosphate is often contaminated with lead in sufficient quantity to impair the assay. This noncompetitive inhibition by ribose 5-phosphate is independent of the competitive inhibition of this substrate as an ATP sink as described by Slabas and Walker. A method for checking reagent purity and removing heavy metal contaminants is described.