The purpose of this experimental work was to determine whether the precancerous loss of rat liver 5-hydroxytryptophan (5-HT) decarboxylase was associated with sequential atrohic alterations preceding hepatocarcinogenesis. To test whether such an association existed, three experimental conditions were employed in which hepato-carcinogenesis was either markedly delayed or prevented. Under all three experimental conditions the losses of 5-HT decarboxylase activity approximated those observed in precancerous livers of rats fed 3′-methyl-4-dimethylaminoazobenzene. It was also shown that the losses were not associated with proliferation stimulated by partial hepatectomy. It was concluded that the precancerous loss of 5-HT decarboxylase activity either (a) was associated with factors other than those involved with sequential atrophic alterations preceding hepatocarcinogenesis or (b) if associated with the atrophic alterations, was a necessary but insufficient reaction leading to malignant transformation. The possible correlation between the activity losses and protein-bound azo dyes was discussed.

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