Determination of Km and V of an enzyme for an unstable substrate and its application to the oxidation of N tau-methylimidazol-3-ylacetaldehyde by aldehyde dehydrogenase.

The method of Storer & Cornish-Bowden [(1974) Biochem. J. 141, 205-209] for determining the lag time in coupled enzyme assays was adapted to enable the kinetic parameters of the second (coupling) enzyme for the intermediate to be calculated. The validity and accuracy of this method of progress-curve analysis was established by comparing the Km value of glucose 6-phosphate dehydrogenase for glucose 6-phosphate generated in situ by the action of glucose phosphate isomerase on fructose 6-phosphate with that determined from initial-rate measurements. The method was applied to the determination of the Km value of ox liver cytoplasmic aldehyde dehydrogenase for N tau-methylimidazol-3-ylacetaldehyde that was generated in situ by the action of plasma amine oxidase on N tau-methylhistamine.