Problems of quantities and units in enzymology.

A knowledge of the molecular structure of an enzyme makes it possible to characterize it by different types of "amount": number (of elementary entities), mass, volume, and amount of substance. Another type of amount is obtained by measuring the rate of reaction of a suitable substrate change elicited by the enzyme in a special defined assay mixture; this way of using the catalytic effect has lead international bodies to different definitions of (kind of) quantity and unit. Their problems are discussed. A coherent scheme of the most important kinds of quantities employed by the enzymologist is presented. The dangers of converting results with one type of assay method to those of another are explained.