α_s1-Casein was deamidated specifically in the moiety of the glutaminyl side chains by transglutaminase. The deamidation was achieved by reversibly blocking the amino groups in substrate protein. The deamidated product was found out to have 80% (11 residues) of the total deamidated glutaminyl residues. No significant conformational changes were observed even after the modifications. Consequently, the solubility in acidic pH regions, especially at pH 5, was increased. On the other hand, the Ca²⁺-sensitivity largely decreased to the extent that nearly all the deamidated product was soluble at 20mM CaCl₂. Deamidation by transglutaminase may be useful as a new method to solubilize insoluble proteins without destroying protein structures.

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