Studies carried out in cell-free rat-liver extracts showed an inhibitory effect of thiamine deficiency on porphyrin synthesis. 2. Extracts of normal and thiamine-deficient rat livers were incubated with the cofactors and substrates of the thiamine pyrophosphate-dependent enzyme, 2-oxoglutarate glyoxylate carboligase. There was a marked inhibition of porphyrin synthesis when the substances were added either individually or together. 3. Thiamine deficiency resulted in a decrease of haemoglobin concentration and catalase activity whereas a rise in tryptophanpyrrolase activity and cytochrome b5 and P450 concentration was observed.
