Pyrroloquinoline quinone (PQQ) is a prosthetic group of various quinoprotein deoxygenases. They catalyze the first step in oxidation of alcohols and sugars in the periplasm of bacteria, contributing to the formation of protonmotive force and, consequently, the formation of ATP. In soluble glucose dehydrogenase (s-GDH),PQQ is responsible for the oxidation of glucose to gluconolactone.sGDH also requires three calcium ions per monomer for its function in a homodimeric form.In order to study the mechanistic details of PQQ enzyme catalysis we study 2-methylquinoline-5, 8-dione as a model of PQQ.Meanwhile, we have found that 2-methylquinoline-5, 8-dione shows an interesting photochemical reactivity. Here, the photochemical reaction of 2-methylquinoline-5, 8-dione and aldehyde is studied.Three products are identified in the photochemical reaction. They are 6-acyl-5,8-dihydroxy-2-methylquinoline, 5,8-dihydroxy-2-methylquinoline, and 6,7-diacyl-5,8-dihydroxy-2-methylquinoline. The mechanism of the formation of these products is discussed.