Abstract
On the basis of n.m.r. and biochemical evidence, the hydroxymethylbilane structure (3) has been deduced for preuroporphyrinogen, the natural substrate for uro'gen III cosynthetase at pH 7·8–8·5 and 37 °C at 0 °C formation of another species, viz the N-alkylpyrrole (2), is indicated, thus reconciling previously conflicting interpretation of these data.