Cellular fructose-P2 aldolase has a derivatized (blocked) NH2 terminus

Fructose-P2 aldolases isolated from vertebrate skeletal muscle have underivatized NH2-terminal proline residues in contrast to most other cytoplasmic proteins which contain alpha-N-acetylated termini. However, if "native" aldolase molecules derived from chicken muscle, rat liver, wheat germ, and the cytosol of spinach leaves are isolated in the presence of phenylmethanesulfonyl fluoride (an inhibitor of serine proteases), they contain blocked and presumably derivatized NH2-terminal residues. When chicken muscle aldolase is isolated in the absence of this protease inhibitor, the derivatized NH2-terminal residue is removed by an endogenous protease(s). The native and modified forms of the enzyme were not distinguished on the basis of catalytic activity, thermal stability, electrophoretic mobility, or subunit molecular weight. Structural analyses of both forms, together with amino acid sequence analysis of the primary translation product encoded for by aldolase mRNA, showed that native muscle aldolase subunits contain a single derivatized methionine NH2-terminal to the proline residue. This form of the enzyme is presumably the one which exists in vivo.