Amino acid sequence of a cyanogen bromide fragment containing the two tryptophanyl residues of lobster arginine kinase (Homarus vulgaris)

Lobster arginine kinase [EC 2.7.3.3] contains 2 tryptophanyl residues and 9 methionyl residues. The whole carboxymethylated protein was first subjected to CNBr cleavage and the resulting fragments were isolated by gel filtration and other experimental approaches. One fragment, CB5, which contains 60 residues including the two tryptophanyl residues and two of the five cysteinyl residues of the protein, was characterized and the results are reported inthis paper. The overall strategy for the establishment of the complete sequence of this fragment was based on the use of three types of peptides: (a) whole cyanogen bromide peptide CB5 which was partially characterized by automatic Edman degradation using a sequencer: 42 steps were performed out of 60 residues, (b) tryptic peptides of CB5, (c) peptides formed by cleavage of S-carboxymethylated arginine kinase (whole protein) at the two tryptophanyl residues with BNPS-skatole. The complete amino acid sequence of the CNBr polypeptide (CB5) which contains the two tryptophanyl residues of the whole protein was established.

Preparation and properties of S-cyano derivatives of creatine kinase

The two reactive thiol groups of rabbit muscle creatine kinase were stoichiometrically reacted with 5,5'-dithio-bis(2-nitrobenzoic acid). In the resulting inactive mixed disulfide derivative they were subsequently substituted with [14C]cyanide, the smallest uncharged thiol-blocking group. The modified enzyme contained 1.6 mol label/mol protein and showed by Ellman's titration and amino acid analysis a concomitant loss of about 0.8 - 0.9-SH group per subunit. This mono-S-cyano derivative of creatine kinase was found 73% as active as the native unmodified protein. It was still able to react in the native state with a variety of thiol reagents with the further blocking of another pair of thiol groups; their substitution once more with cyanide resulted in the bis-S-cyano derivative of creatine kinase, which lost 2 thiol groups per subunit and had about 50% of the original catalytic activity. It is concluded that the four cyanylated thiol groups are not required for the catalytic activity of creatine kinase and the cyanoprotein derivatives described are shown to be useful tools for some interesting investigations related to this enzyme.

[Isolation of peptide containing essential tyrosine from lobster arginine kinase]

The essential tyrosine residue of Lobster muscle arginine kinase, which is part of an antigenic determinant, has been modified by tetranitromethane. Cleavage of the S-carboxymethylated nitrated enzyme with cyanogen bromide gives rise to eight peptides, one of which containing the labelled essential tyrosyl group. Ion exchange chromatography on sulfoethyl-Sephadex C-25 in urea medium has been used with success for isolation and purification of the nitrated peptide. From its amino acid composition and end groups structure this peptide is the N-terminal fragment of the protein.

The tyrosyl residues in creatine kinase. Modification by iodine

The effect of the iodination of tyrosyl residues in creatine kinase from rabbit muscle has been investigated at alkaline pH after reversible masking of the reactive thiol groups. The conversion of 4-5 tyrosyl residues to monoiodotyrosines as measured by spectrotitration and by radioactive iodine labelling resulted in almost total loss of enzymic activity. The modified enzyme was unable to bind its nucleotide substrates but no significant conformational change was revealed by optical rotatory dispersion or Stokes radius measurements. However, change in the reactivity of some non-essential thiol groups, presumably those located near the active thiol groups, was observed.