Rabbit muscle phosphofructokinase uniformly carboxymethylated with iodo[2-14C]acetate consists of subunits with a molecular weight of 80 000 +/- 5000. The subunit polypeptide chain contains 16 and 52 residues respectively of cysteine and arginine and, contrary to previous results, peptide mapping experiments gave no indication that phosphofructokinase chains yield fewer than the expected numbers of cysteine and arginine containing peptides. To test further for the possible occurrence of repeat sequences within a single subunit chain, cysteine-containing peptides were isolated and sequenced from tryptic and thermolytic digests of s-[2-14C]carboxymethylated phosphofructokinase. In all, 15 different cysteine sequences (comprising a total of 104 residues) were identified, showing that not more than one of an expected 16 cysteine-containing sequences is repeated, and that the subunits of phosphofructokinase are of unique sequence along their entire length. The near quantitative isolation of several cysteine-containing peptides shows further that all subunits are of similar if not identical sequence.

An investigation to determine the number of proteins comprising the keratin of the chick down feather was carried out using anion‐exchange chromatography of the carboxymethylated derivatives of reduced feather proteins, followed by examination of discrete protein fractions by polyacrylamide gel electrophoresis at pH 9.5 and pH 2.7 and by amino acid analysis. Twenty two different proteinswere identified by these techniques and it is likely that at least nineteen are closely related to one another in terms of primary structure. The heterogenteity observed was not the result of incomplete alkylation or of non‐spceific reactions during the carboxymethylation procedure nor did it arise during the procedures used to separate or analyse the carboxymethylated keratin.