HOW THE ATMOSPHERE AND THE PRESENCE OF SUBSTRATE AFFECT THE PHOTO ANDNON-PHOTOINACTIVATION OF HEME ENZYMES BY UROPORPHYRIN-I

1. The effect of URO I on the activity of ALA-D, PBGase, deaminase and URO-D, both in aerobiosis and anaerobiosis, was studied. 2. Photoinac tivation of the enzymes was much lower in an anaerobic than in an aero bic atmosphere. 3. Dark inactivation in the absence of oxygen was lowe r than its presence. 4. Preincubation in the presence of ALA or PBG pr otected the enzymic activity of ALA-D, PBGase and deaminase against UR O I-inactivation both under u.v. light and in the dark. 5. Photoinacti vating action of URO I would be mediated by reactive oxygen species ge nerated by the excited porphyrin after its absorption of light. Dark i nactivation, in aerobiosis, can also be partly mediated by amino acid oxidation, although to a lesser extent than that observed under u.v. l ight.