Probing the active site of pea seedlings amine oxidase with optical antipodes of sedamine alkaloids.

Interactions of pea seedlings amine oxidase (PSAO, EC 1.4.3.6) with sedamine derivatives were studied. All compounds exhibited a competitive inhibition with the inhibition constants in the range 0.03-1.0 mM. The inhibition effect increased in the order allosedamine < sedamine << norallosedamine < norsedamine. The nor-derivatives are about five-fold stronger inhibitors and the allo-isomers are slightly weaker inhibitors than the others. Interestingly, the (-)-diastereomers of the studied sedamines were considerably stronger inhibitors than the (+)-antipodes. Absorption spectroscopy was used to differentiate between two known groups of competitive inhibitors of PSAO. A representative of substrate analogues, 1,5-diamino-3-pentanone, bleached the spectrum of the TPQ cofactor producing a very stable intermediate of the enzyme catalytic cycle that was only slowly converted to the product. On the other hand, the alkaloids did not perturb the spectrum of TPQ so they may interact with some other residue near the active site.