1. The a and ,B subforms of aspartate aminotransferase were purified from pig heart. 2. The
a subform contained 2mol of pyridoxal 5'-phosphate. The apo-(a subform) could be fully
reactived by combination with 2mol of cofactor. 3. The protein fluorescence of the apo-
(a subform) decreased non-linearly with increase in enzyme activity and concentration of
bound cofactor. 4. It is concluded that the enzyme activity/mol ofbound cofactor is largely
independent of the number ofcofactors bound to the dimer. 5. The /Jsubformhad approximately
half the specific enzyme activity of the a subform, and contained an average of one
active pyridoxal 5'-phosphate molecule per molecule, which could be removed by glutamate,
and another inactive cofactor which could only be removed with NaOH. 6. On
recombination with pyridoxal 5'-phosphate the protein fluorescence of the apo-(fl subform)
decreased linearly, showing that each dimeric enzyme molecule contained one active
and one inactive bound cofactor. 7. The results are not consistent with a flip-flop mechanism
for this enzyme
