Identification of cysteine residues at the 10-pmol level by carboxamidomethylation of protein bound to sequencer membrane supports.

A procedure is described for the in situ carboxamidomethylation of cystine/cysteine residues in protein samples of as little as 10 pmol, prior to automated protein sequence analysis. Previous in situ methods for the modification of cysteines are limited to proteins available in quantities greater than 100 pmol due to contaminants which interfere with HPLC identification of phenylthiohydantoin amino acids, and cannot be performed on polyvinylidenedifluoride (PVDF)-bound samples. In our procedure, protein samples, immobilized on either PVDF- or polybrene-treated glass filters, are reduced with tributylphosphine followed by alkylation with iodoacetamide prior to automated sequence analysis. Carboxamidomethylcysteine is formed in high yield with no discernable side reactions in standard proteins (insulin, human transferrin, lysozyme) or experimental samples. Both initial and repetitive yields of carboxamidomethylated proteins were either comparable to or better than nonalkylated proteins. No apparent increase in background nor any sequence preview due to partial amino-terminal alkylation was observed. The carboxamidomethylation procedure described here successfully overcomes the limitations of available methods for reduction and alkylation of less than 100 pmol of protein directly on sequencer membrane supports.