Effect of calcium on subcellular distribution of peroxidases

About 40% of the peroxidase activity extracted from hypocotyl hooks of etiolated Cucurbita pepo was pelletable at 20000 g. The activity in the pellet was partially solubilized by the addition of 5 mM EGTA. This effect of EGTA was reversed by Ca²⁺, but not by Mg²⁺. Reassociation of EGTA-solubilized peroxidases to 4 cellular fractions obtained by centrifugation on discontinuous sucrose gradients was assayed. It appeared that the enzymes could be linked to ribosomes or RNP particles through Ca²⁺. Two fractions, identified as smooth and rough endoplasmic reticulum, also bound peroxidases and, in this case, the Ca²⁺-mediated binding involved a loss of the enzyme activity. The fraction containing mitochondria and plasmalemma exhibited a slight binding capacity. Isoelectric focusing in thin layer polyacrylamide gel showed that only 5 out of the 10 isoperoxidases present in hypocotyl hooks had their pelletability level changed by Ca²⁺.