[N-terminal sequence of a porphobilinogen synthase]

The N-terminal sequence of porphobilinogen-synthase (EC 4.2.1.24) from bovine liver up to position 44 is given. After tryptic hydrolysis two N-terminal peptides with identical amino acid composition were isolated in the ratio 1:1. One peptide was blocked whereas the other one started with methionine and showed the same primary structure which had been estimated by automatic degradation of the enzyme. Four of the eight subunits of PBG-S have free methionine as N-terminal amino acid whereas the other half is blocked.