INVOLVEMENT OF CALCIUM IN THE INHIBITION OF ENDOPOLYGALACTURONASE ACTIVITY IN EPIDERMIS CELL-WALL OF LINUM-USITATISSIMUM

There are different hypotheses to interpret the inhibitory effect of C a2+ on the hydrolysis of galacturonans by endoPG (endopolygdacturonase ). In studying the rate of hydrolysis of PGA (polygalacturonic acids) or of LMP (low methylesterified pectins) by endoPG irt vitro, in the p resence of various concentrations of several cations including Ca2+, w e have favoured the ''steric hindrance'' hypothesis. According to this hypothesis, the inhibitory effect of calcium works via the formation of calcium bridges cross-linking galacturonan polysaccharides with one another, thus decreasing their enzymic hydrolysis and/or the mobility of the enzyme and its access to the substrate. Using conventional sub traction methods (treatments by cycle hexane diamine tetra-acetate CDT A-Na-2 or boiling water) we have determined compositional features of the different regions parts of the cell wall of flax seedling hypocoty l epidermis, after a glutaraldehyde-fixation of the plant samples. We have then predicted the sensitivity of these different wall areas to e ndoPG hydrolysis, without or with calcium, according to the ''steric h indrance'' hypothesis. The results obtained, in a kinetic study of end oPG hydrolysis of the walls of plant samples identical to those subjec ted to the CDTA-Na-2 and boiling water treatments, were in excellent a greement with predictions. This not only strengthens the validity of t he ''steric hindrance'' hypothesis, but it is also a demonstration of the efficacy of a kinetic study of cell wall hydrolysis by endoPG, to determine the types of bonds involved in pectic molecule associations in the wall.