Effect of chemical modifications on the gushing-inducing activity of a hydrophobic protein produced by a Nigrospora sp.

A beer-gushing inducing protein (NGF) produced by Nigrospora sp. No. 207 was found to be characterized by a relatively higher content of hydrophobic amino acid residues, a considerably high content (about 10%) of half-cystine residue, and the lack of tyrosine, tryptophan, methionine and histidine. Cleavage of disulfide bonds in NGF, either oxidative or reductive, resulted in complete loss of the gushing-inducing activity, indicating that the disulfide bonds are essential for exhibiting the activity. Modification of amino groups in NGF with maleic anhydride or O-methylisourea did not affect the gushing activity, whereas modification with trinitrobenzene sulfonate reduced the activity to some extent. Modification of arginine-guanidino groups in NGF with phenylglyoxal markedly reduced the gushing activity. When the free carboxyl groups of NGF were modified by incorporating glycine or tyrosine, the gushing activity was also drastically destroyed, implying that the acidic groups of the NGF protein are important for the gushing activity. It appeared that maintenance of a definite molecular conformation is essential for the gushing-inducing activity of this hydrophobic protein.