3 The differing pH. optima for PAL and
TAL at saturating substrate concentrations may be expected since they are determined by
the ionization state of groups of the enzyme-substrate complex which varies with the substrate.32
The question of how many enzymes is therefore not settled and is likely to remain
unanswered until more work is done with highly purified preparations. If a generalization
can be made it is that some PAL preparations display catalytic activity towards tyrosine.