Ca2+ was found to be an inhibitor of plant invertases in vitro. But in contrast bacterial invertase (Arthrobacter) was not affected. Inhibition of sugarcane leaf sheath invertase was competitive, reversed by dialysis and markedly increased by preincubation of the enzyme with Ca2+ before addition of substrate. The apparent inhibition constant (Ki) and the apparent number of Ca2+ (n) bound per enzyme molecule were found to be 79 mM and 0.85 respectively. The overall reaction was endothermic. Consequently, Ca2+ is postulated to act by a reversible binding to the enzyme.
