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Citric acid in the presence of Zn(++) at pH 8.2 is catalyzed to oxaloacetate by the enzyme, citrate lyase. Oxaloacetate in the presence of malate dehydrogenase and reduced nicotinamide adenine dinucleotide (NADH) is reduced to malate (II). By measuring the disappearance of the light-absorbing NADH, the citric acid concentration in the reaction mixture can be determined.(Nielsen TT: A method for enzymatic determination of citrate in serum and urine. Scand J Clin Lab Invest 1976;36:513-519)
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