The kinetics of hydrolysis of some synthetic substrates containing neutral hydrophilic groups by pig pepsin and chicken liver cathepsin D.

1. Several peptides containing either of the sequences -Phe(NO2)-Trp- and -Phe(NO2)-Phe- and an uncharged hydrophilic group were synthesized, and the steady-state kinetics of their hydrolysis by pig pepsin (EC 3.4.23.1) and chicken liver cathepsin D (EC 3.4.23.5) were determined. Despite the presence of a hydrophilic group to increase substrate solubility, it was not possible to achieve the condition [S]0 much greater than Km, and, in some cases, only values of kcat./Km could be determined by measuring the first-order rate constant when [S]0 much less than Km. 2. Occupancy of the P2 and P3 sites considerably enhanced the specificity constant, and alanine was more effective than glycine at site P2. 3. The specificity constants for the hydrolysis by pepsin of those substrates in the present series that contain an amino acid residue at site P3 are considerably lower than for comparable substrates containing a cationic group. This difference does not apply to cathepsin D. 4. Hydrolyses with cathepsin D commonly exhibited a lag phase, and a possible explanation for this is given.