The activities of orotate phosphoribosyl transferase (OATase) and orotidine 5′-monophosphate decarboxylase (OMPase) in the 3683, 3924A, 7288C, 5123D, and 7800 Morris hepatomas were higher than those of normal liver. However, neither of these enzyme activities correlated with the growth rates of the hepatomas.

OATase and OMPase activities in 24-hr regenerating livers were 149 to 169% (and after 48 hr were 218 to 244%) those of livers from sham-operated rats. OATase and OMPase activities in livers from sham-operated rats were 94 to 125% those of normal liver. Fasting, or fasting and refeeding had no effect on either enzyme.

The Km's for OATase in liver and hepatomas were 1.8 to 3.6 × 10-6m for orotic acid, and the Km's for OMPase were 6.7 to 8.8 × 10-7m for orotidine monophosphate. The pH for maximum activity was 7.1 for OATase and 6.2 for OMPase in liver or the hepatomas.

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