Transformation of neutral to alkaline fructose 1,6-bisphosphatase. Converting enzyme activity in the large-particle fraction from rabbit liver

A liver particle fraction containing lysosomes catalyzes the conversion of native rabbit liver fructose 1,6-bisphosphatase (EC 3.1.3.11), having a neutral pH optimum, to a modified form with an alkaline pH optimum. The “converting enzyme” activity is partially recovered with the membranes from disrupted particles, and is also detected in “intact” particles isolated and maintained in isotonic buffered sucrose. The converting enzyme activity associated with the membrane fraction is expressed at pH 6.5, but not at pH 4.5, although activity at the lower pH appears when the enzyme is released from the membranes with Triton X-100. In contrast, proteolytic activity as measured with peptide and protein substrates is maximal at pH 5.0 or below, and is the same for the membrane-bound or solubilized proteases. The results suggest that a specific converting enzyme, at least partially associated with a particle (possibly lysosomal) membrane, is responsible for the modification of fructose bisphosphatase and the change in its catalytic properties.