The relationship of structure to activity in plant-growth-regulating compounds. 2. Growth-regulating activity of substituted amides of 2,4-dichlorophenoxyacetic acid

involving alternate ionization pathways is used then the significance of the observed rate constant for the breakdown of the enzymoe-substrate com- plex is altered.In fact the observed rate constant equals the true rate constant multiplied by the factor KAIKl and incorporation of this in equation ( 12) makes it identical with Alberty & Massey's formula.Hence their procedure for calculating K1 and K2 from the pH dependence of V is valid on either model of enzyme activity.SUMMARY 1.The theory of kinetics of the reaction between chloroacetamide and amino thiols developed by Lindley (1960) has been further investigated.2. The reaction between four cysteine peptides and chloroacetamide has been studied experi- mentally.3. For three of the peptides it appears probable that pK8, > pK.,2+; for the fourth peptide, glutathione, pK,,3+ > pK,-.4. Some general aspects of the determination of pK of complex ions by kinetic methods are dis- cussed and their relevance to some aspects of enzyme kinetics is pointed out.