Formation of cobalt protoporphyrin by chicken hepatocytes in culture. Relationship to decrease of 5-aminolaevulinate synthase caused by cobalt.

Cobalt protoporphyrin generated from 5-amino[4-14C]laevulinate by homogenates or primary cultures of chick embryo liver exposed to CoCl2 was found to be radioactivity unextractable by acid/acetone, when extra protein was added. The activity of ferrochelatase was required for formation of cobalt protoporphyrin since inhibition of ferrochelatase with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (in the presence of cycloheximide) inhibited formation of cobalt protoporphyrin and resulted in accumulation of protoporphyrin. Cobalt protoporphyrin was detected spectrophotometrically in hepatocyte cultures exposed to the combination of 2-allyl-2-isopropylacetamide and CoCl2: (1) as the pyridine haemochrome of the protein pellet remaining after acid-acetone extraction of the cells, or (2) as the material extracted from the protein pellet with acetic acid-pyridine-chloroform. The amount of cobalt protoporphyrin increased with increasing CoCl2 concentration as cellular haem declined. The decrease in haem was about equal to the amount of cobalt protoporphyrin that accumulated. 2-Allyl-2-isopropylacetamide and polychlorinated biphenyls were both powerful inducers of 5-aminolaevulinate synthase. The former led to protoporphyrin accumulation, whereas with the latter, uroporphyrin accumulated, probably due to a concomitant decrease in activity of uroporphyrinogen decarboxylase. The decrease in activity of 5-aminolaevulinate synthase produced by administration of CoCl2 was greater after treatment with 2-allyl-2-isopropylacetamide than after treatment with allylisopropylacetamide and 3,4,3',4'-tetrachlorobiphenyl. We conclude: (a) that cobalt protoporphyrin is readily formed in cultured hepatocytes, and (b) that its formation accounts for the action of cobalt on 5-aminolaevulinate synthase.