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Ultraviolet laser light produces in pyridoxal-5'-phosphate imines in free solution and in aspartate aminotransferase interesting photochemical reactions. In the amino acid imines in nonaqueous solvents, a proton transfer from the 3-OH group to the imine nitrogen occurs in the excited singlet state. The slow back transfer produces a long-lived transient state which decays with first-order kinetics. Under conditions which do not favor this transfer, as with an ionized group in the aspartate aminotransferase enzyme, the cofactor crosses over into a triplet state on excitation. This state decays with complicated kinetics. When aspartate aminotransferase binds aspartate, a substrate, a UV-induced band at 500 nm occurs which is not present with α-methylaspartate, an inhibitor. The decay of this band is largely first order with a rate constant of 2600 s-1. This absorption appears to originate from the photoinduced key -quinoid intermediate structure in the enzyme's mechanism.