Essential sulfhydryl groups in diamine oxidase from Euphorbia characias latex

Diamine oxidase from Euphorbia characias latex contains two sulfhydryl groups per mole of dimeric enzyme. The sulfhydryl groups are unreactive in the native enzyme but can be readily titrated by 4,4'-dithiodipyridine after protein denaturation, or anaerobically in the presence of the amine substrate. In the presence of both substrates (diamine and oxygen) they react sluggishly. The sulfhydryl groups show different reactivity toward various reagents, but in every case their modification inhibits catalytic activity. The insensitivity of the native enzyme to specific reagents suggests that the sulfhydryl groups are positioned in the interior of the protein and shielded from the solvent. Their reactivity in the presence of the amine substrate could be attributed to a conformational change occurring upon substrate binding or after substrate oxidation.