Printed in Great Britain The Use of the Direct Linear Plot for Determining Initial Velocities

A new method is described for estimating initial velocities of enzyme-catalysed reactions. It is simple to apply either graphically or numerically, and is particularly appropriate for experiments in which the initial straight part of the progress curve is very short or non-existent. It requires no more knowledge than is readily available about the details ofthe system, such as the extent ofreaction at equilibrium, the rate ofenzyme inactivation, the nature of product inhibition etc., unlike some other methods of analysing progress curves, which are often invalidated by small errors in the defining assumptions. The direct linear plot (Eisenthal & Cornish-Bowden, 1974) provides a simple method ofanalysing initial velocities measured in enzyme kinetic experi-ments, and its statistical treatment (Cornish-Bowden & Eisenthal, 1974) is both easier to understand and less dependent on arbitrary assumptions than the widely used method of least squares. But effective analysis of initial velocities satisfies only one-half of the problem, because it presupposes a reliable method of estimating initial velocities from progress curves. This is difficult to do accurately unless there is a significant linear period at the beginning of each curve, and any systematic error that results from subjective bias is unlikely to be detected by subsequent statistical treatment. There are three principal ways of estimating initial velocities, (i) estimation by eye with ruler and pencil, (ii) methods that use the integrated form of the rat