Diffusion and simultaneous chemical reactions: III. The degree of localization achieved in cytochemical staining procedures

Further consideration is given to assessing the degree of precision achieved in the localization of enzymes in cells by chemical staining techniques. Procedures discussed are those in which the enzyme acts on a substrate by a zero-order reaction, producing an intermediate which is then converted by an irreversible first-order reaction (velocity constantk) into a non-diffusible colored product. A relationship is obtained which permits the degree of localization for this type of process to be easily calculated. This localization factor is independent of the activity per unit volume of the site; it increases with the size of the site and, for experimental time ranges, is virtually independent of the time of staining. It is shown that, under usual circumstances, satisfactory localization is achieved in sites of radius 1 μ ifk/D is of the order 10⁹ cm⁻² and that when this ratio has the value 10¹¹ cm⁻² localization is very good.