A NEW ELECTROPHORETIC VARIANT OF ALPHA-SUBUNIT OF NA+/K+-ATPASE FROM THE SUBMANDIBULAR-GLAND OF RATS

The α catalytic subunits of Na+ K+-ATPase were isolated from the kidney and brain of rats (α1 and α2, respectively). The antisera raised against these subunits were used as probes to analyze the isoform of catalytic subunits of Na+ K+-ATPase in various tissues of rats. Of 27 rat tissues examined, most had a catalytic subunit identical to α1 but some, such as the nervous and muscle tissues, had both α1 and α2 isoforms as judged by their reactivities to antisera and their electrophoretic mobility. We found that the submandibular gland contained a new electrophoretic variant of immunoreactive α subunit (designated α(S) in this report) in addition to α1 identical to those found in kidney and brain. The new variant, α(S), strongly cross-reacted with anti-α1 antiserum, but to a lesser extent with anti-α2 antiserum. The α(S) had a molecular mass which was found to be slightly less (approx. 90 kDa) than brain and kidney α1. We examined whether or not the α(S) is formed by proteolytic cleavage of α subunits during preparation and concluded that this is not the case. The α(S) reacted with [γ-32P]ATP, resulting in the formation of radioactive α subunit which was stabilized by 2 mM ouabain but which was labile in the presence of 70 mM potassium chloride. Since N-terminal amino acid sequence of α(S) protein [G()DKY()PAAVS] corresponds exactly and uniquely with the sequence of the α1 chain between residues 1 and 11, it is very probable that α(S) protein originated from α1 protein following the post-translational processing. © 1990.