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The accumulation of an inactive, immunologically crossreactive form of fructose-1,6-bisphosphate aldolase (EC 3.1.3.11) in livers of fasted rabbits has now been related to limited proteolysis at the COOH terminus. The extent of modification of this region of the molecule, determined by analysis of tyrosine residues in the peptides released by digestion with subtilisin, agrees with the observed decrease in the specific activity of the enzyme purified from livers of fasted rabbits. The following evidence supports the conclusion that the modified form is produced in vivo and not during the isolation of the enzyme from the liver homogenates: (i) liver homogenates prepared in isotonic sucrose contained negligible amounts of soluble lysosomal proteinases; (ii) the decreased aldolase activity after fasting was observed in the homogenates and no change in aldolase activity occurred when the homogenates were incubated for 2 hr at 37 degrees C; (iii) the modified enzyme was also isolated from the livers of fasted rabbits when leupeptin was injected intraportally before the animals were sacrificed or when the inhibitor was added to the homogenization solution. On the other hand, homogenization of livers in hypotonic medium resulted in release of lysosomal proteinases and also in decreases in catalytic activity and COOH-terminal modification of liver aldolase, similar to those observed in livers from fasted rabbits. We attribute the changes in activity and structure of aldolase isolated from livers of fasted rabbits to the action in vivo of cathepsin M.

Characterization of the inactive form of fructose-1,6-bisphosphate aldolase isolated from livers of fasted rabbits.