The Enzymatic Synthesis of Uroporphyrins from Porphobilinogen

It has been reported (1) and confirmed (2) that the production of uroporphyrin III from porphobilinogen, either by heating in acid solution or enzymatically by hemolyzed avian erythrocytes, is accompanied by the formation of formaldehyde from the aminomethyl group of the pyrrole. These observations have been cited as evidence in support of two hypotheses on the mechanism of uroporphyrin III formation (1, 2). The hypothesis of Shemin et al. (1) requires the accumulation of 1 mole of formaldehyde per mole of uroporphyrin III formed. The availability of purified enzyme preparations which catalyze the formation of uroporphyrinogen I or III from porphobilinogen (3, 4) has permitted a quantitative investigation of formaldehyde accumulation during the enzymatic synthesis of these porphyrinogens; under these conditions interpretation of the results is least complicated by possible side reactions. Furthermore, a comparison of formaldehyde accumulation during the enzymatic formation of the I and III isomers of uroporphyrin should provide evidence regarding the involvement of formaldehyde specifically in synthesis of the III isomer. In addition, it has been reported (3) that formaldehyde accelerates the nonenzymatic synthesis of porphyrins from porphobilinogen although it appears to inhibit the enzymatic reactions. Some of the present investigations were designed to determine whether formaldehyde can be a reactant in the formation of porphyrins from porphobilinogen. This information is also pertinent to an evaluation of a proposal by Treibs and Ott (5) and the hypothesis of Cookson and Rimington (6) ; both of these suggestions require the liberation of formaldehyde from the aminomethyl group of porphobilinogen and the subsequent utilization of this formaldehyde for the formation of uroporphyrinogen III. The data presented here indicate that formaldehyde is neither a stoichiometric by-product of the enzymatic formation of uroporphyrinogen III from porphobilinogen nor a reactant in this process.