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The isopenicillin N synthase (cyclase) of Streptomyces lactamdurans (syn. Nocardia lactamdurans) has been purified to near homogeneity as judged by SDS-PAGE and isoelectric focusing. This enzyme catalyses the oxidative cyclization of the tripeptide δ-(l-α-aminoadipyl)-l-cysteinyl-d-valine to isopenicillin N. The enzyme required DTT, Fe2+ and oxygen and it was greatly stimulated by ascorbic acid. It was strongly inhibited by Co2+, Zn2+ and Mn2+. Optimal pH and temperature were 7·0 and 25 °C (with the assay conditions used), respectively. The apparent K m of isopenicillin N synthase for δ-(l-α-aminoadipyl)-l-cysteinyl-d-valine was 0·18 mm. The enzyme is a monomer with an M r of 26500 ± 1000 and a pI of 6.55.
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