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The ligand-binding kinetics of the isolated component from trout (Salmo irideus) blood has been extensively investigated ; it appears to be fully consistent with their equilibrium behaviour, described previously. At alkaline pH (z 7 4 , in a region where the Root effect is not operative, comparison of the kinetics of the various components (Hb trout I, 11 and IV) shows considerable similarities between them. For Hb trout I (and 11) the time course of the reaction with both 0, and CO is practically pH independent. On the other hand Hb trout IV, characterized by a Root effect, displays a strong dependence on pH of the kinetics of ligand binding. Thus in the pH region where the Root effect becomes operative there is a large change of the character, as well as of the speed, of the kinetics with both 0, and CO.The Root effect characteristic of Hb trout IV appears to originate, from a kinetic point of view, in a pH-dependent change of both the "on" and "off" constants, although a t least for 0, the effect of pH on the ''off' kinetic constants is much more marked.

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