BIOSYNTHESIS OF PORPHYRINS AND RELATED MACROCYCLES .41. FATE OF OXYGEN-ATOMS AS PRECORRIN-2 CARRYING 8 LABELED CARBOXYL GROUPS (13C18O2H) IS ENZYMATICALLY CONVERTED INTO COBYRINIC ACID

5-Amino[1,4-(C2)-C-13]laevulinic acid and 5-amino[1-C-13]laevulinic ac id are synthesised and all three O-16 atoms of the latter are exchange d for O-18. The C-13, O-18-labelled material is then converted in vitr o into precorrin-2 2 by the combined action of four genetically overpr oduced enzymes. The product is isolated in its aromatised form, sirohy drochlorin 10 and C-13-NMR shows that all 8 carboxyl groups of 10 reta in both oxygen atoms throughout the biosynthesis. A cell-free enzyme p reparation from Propionibacterium shermanii converts the C-13, O-18-la belled sirohydrochlorin 10 via 2 into cobyrinic acid 4, a late precurs or of vitamin B12. C-13-NMR proves that 6 carboxyl groups of cobyrinic acid (b-g, inclusive) retain both oxygen atoms whereas the a-carboxyl group undergoes specific loss of one labelled oxygen atom.