Heterogeneity of bovine seminal ribonuclease

Bovine seminal ribonuclease, a dimeric protein found to be homogeneous by several standard criteria of purity, is heterogeneous when analyzed by ion-exchange chromatography on (carboxymethyl)cellulose. Three increasingly cationic subforms can be separated. The heterogeneity is due to the presence of two types of subunits, alpha and beta, which make up three isoenzymic dimers: alpha 2, beta 2, and alpha beta. Deamidation reactions can convert the most cationic beta 2 subform into the alpha beta subform, which in turn can be converted into stable alpha 2 subform. These conversions involve the hydrolysis of 2 mol of differentially labile amide groups per mol of protein. The ratios alpha 2: alpha beta: beta 2 are constant in all preparations of seminal ribonuclease tested; they are independent of the purification procedure as well as of the biological source of the enzyme (seminal plasma or seminal vesicles). These results indicate that deamidations occur in vivo before the protein is secreted from the seminal glands. They also suggest that heterogeneity of seminal ribonuclease reflects a physiological need of distinct molecular forms of enzyme or, alternatively, a process which leads to the aging of the protein.