title: The Kinetics of Enzyme Mixtures.
abstract: Even purified enzyme preparations are often heterogeneous. For example, preparations of aspartate aminotransferase or cytochrome oxidase can consist of several different forms of the enzyme. For this reason we consider how different the kinetics of the reactions catalysed by a mixture of forms of an enzyme must be to provide some indication of the characteristics of the species present. Based on the standard Michaelis-Menten model, we show that if the Michaelis constants (Km) of two isoforms differ by a factor of at least 20 the steady-state kinetics can be used to characterise the mixture. However, even if heterogeneity is reflected in the kinetic data, the proportions of the different forms of the enzyme cannot be estimated from the kinetic data alone. Consequently, the heterogeneity of enzyme preparations is rarely reflected in measurements of their steady-state kinetics unless the species present have significantly different kinetic properties. This has two implications: (1) it is difficult, but not impossible, to detect molecular heterogeneity using kinetic data and (2) even when it is possible, a considerable quantity of high quality data is required.
authors.affiliation: Deviot Institute, Deviot, Tasmania 7275, Australia,; School of Human Life Sciences, University of Tasmania, Launceston, Tasmania 7250, Australia.
authors.forename: Simon
authors.initials: S
authors.lastname: Brown
authors.affiliation: Royal College of Medicine Perak, University Kuala Lumpur, Ipoh, Malaysia.
authors.forename: Noorzaid
authors.initials: N
authors.lastname: Muhamad
authors.affiliation: Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand.
authors.forename: Kevin
authors.initials: K
authors.lastname: C Pedley
authors.affiliation: Deviot Institute, Deviot, Tasmania 7275, Australia,; Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand; Faculty of Medicine, Health and Molecular Sciences, James Cook University, Cairns, Queensland 4870, Australia.
authors.forename: David
authors.initials: D
authors.lastname: C Simcock
keywords: aspartate aminotransferase; enzyme kinetics; heterogeneity; isozyme