VARIABLE HEAT-STABILITY FOR MULTIPLE FORMS OF PECTIN METHYLESTERASE FROM CITRUS TISSUE-CULTURE CELLS

Six forms of pectin methylesterase from Citrus sinensis var. Valencia juice sec derived tissue culture cells have been partially purified by anion-exchange and affinity (heparin and concanavalin A) chromatograp hy. Data on thermal stability and thermal denaturation are provided. O ne of these forms (PME 3b) retained 29% activity after being incubated at 95 degrees C for 30 s. Two other forms also retained activity at 8 0-90 degrees C. The most active form, accounting for 93.6% of total ac tivity at 30 degrees C, was completely inactivated after 2 min of incu bation at 80 degrees C. The most heat stable form, PME 3b, has a K-m o f 0.0309 mg mL(-1) and a V-max of 1.4596 mu equiv min(-1). PME 3b has a pH optimum of 7.0 and is glycosylated. Denaturing electrophoresis an d native gel filtration chromatography suggest molecular weights betwe en 34 000 and 47 000 for the partially purified PMEs. The molecular we ight of PME 3b estimated by LDS-PAGE was 37 500 and that estimated by gel filtration chromatography was 40 600.