Purification and properties of the age-related protein (ARPB) detected in human brain: comparison with human brain calmodulin

ARPB is an age-related protein in the human brain which is present in individuals younger than approximately 40 years of age, but disappears or becomes remarkably decreased in individuals above this age (Yasuda, T. and Kishi, K. (1985) Proc. Japan Acad. 61B, 273-276). ARPB was isolated from four different cerebra obtained from subjects 0, 15, 35 and 37 years old, and purified to an electrophoretically homogeneous state. Its molecular weight was estimated to be 17,000 and 36,000-38,000 by SDS-polyacrylamide gel electrophoresis and gel filtration, respectively. The amino acid composition of the purified ARPB, containing an excess of acidic amino acid residues (about 33%), proved to be very similar to that of calmodulin. ARPB exhibited some of the properties characteristic of calmodulin, such as a Ca2+-dependent mobility change upon electrophoresis, and activation of calmodulin-deficient phosphodiesterase activity in a Ca2+-dependent manner. However, it was possible to distinguish ARPB and human calmodulin with regard to their kinetic parameters for the activation reaction of phosphodiesterase activity, despite a close similarity in their reaction mode. It can be concluded that ARPB belongs to one of the calmodulin group proteins, although it remains unknown whether ARPB is identical to calmodulin.