Quantitative description of absorption spectra of a pyridoxal phosphate-dependent enzyme using lognormal distribution curves.

Ultraviolet-visible absorption spectra of cytosolic aspartate aminotransferase of pig hearts have been analyzed by resolution with lognormal distribution curves. These have been compared with spectra of reference Schiff bases of pyridoxal 5'-phosphate. Spectra of the free enzyme in two different states of protonation and of complexes with monoanions, dicarboxylates, the substrates L-glutamate, L-aspartate, and L-erythro-3-hydroxyaspartate, and the quasi-substrate 2-methylaspartate have been analyzed. Relative amounts of three tautomeric species have been estimated, as have amounts of various enzyme-substrate intermediates. Bandshape parameters which can be used as a guide to analysis of spectra of other pyridoxal phosphate-dependent enzymes are tabulated. Some formation constants and pKa values, which were evaluated at the same time as the spectra of the complexes, are also reported.