The finding of a powerful inhibitor of pectin methylesterase in ripe kiwi fruit is reported. The inhibitor was revealed to be a glycoprotein. It was purified to homogeneity and found to have a molecular mass of about 28 kDa, as estimated by gel filtration chromatography, SDSjPAGE and analytical ultracentrifugation. The sugar portion is composed of galactose, arabinose and rhamnose, the latter being much less represented. The amino acid composition showed a very high content of acidic residues compared to basic ones, which is the reason for the very low isoelectric point of the protein (less than 3.5). The kind of inhibition on kiwi pectin methylesterase was found to be competitive with an apparent Ki of 0.22 pM, using citrus pectin as a substrate. Moreover, the inhibitor is effective in inhibiting pectin methylesterase in the pH range 3.5 -7.5. Kiwi inhibitor appears to be specific for pectin methylesterase, inasmuch as it was found to be ineffective against other polysaccharidedegrading enzymes, such as polygalacturonase and amylase. Conversely, it appears to be completely aspecific as far as the pectin methylesterase source is concerned. In fact, it was found to inhibit this enzyme effectively from all the sources we assayed, i. e. orange, tomato, apple, banana, potato.The enzyme pectin methylesterase (PME) is a ubiquitous plant enzyme believed to play a fundamental role in vegetable cell metabolism [l]. The protein is cell-wall-bound [2] and is involved in the ripening process of fruit [3] as well as in cell wall extension during cell growth. In the ripening mechanism, PME acts by producing pectin with a low degree of methylation which is then subject to further enzymatic cleavage by polygalacturonase [4]. The role played by PME in cell wall extension consists essentially in generating a local pH decrease which, in turn, activates enzymes involved in wall autolysis and extension [5 -71. The control of this growth mechanism, therefore, relies on opposite pH sensitivities of the enzymes involved in the process: the PME, having an optimum pH around neutrality, decreases its activity as a consequence of the pH decrease due to its own action. Conversely, the local low pH activates the glycohydrolases, which produce a limited cell wall autolysis, and the glycosyltransferases, which build up and elongate the cell wall. As a consequence of the cell wall reconstitution, a pH increase occurs that activates PME, and so on.Control of the enzymatic activity of PME is also of prime importance in biotechnological processes which concern the production and storage of fruit juices and purees. In fact, it is well known [4, 81 that PME activity changes the texture of fruit products. For this reason, product storage at very low temperature is required or a pasteurization process at high temperature must be employed to inactivate the PME in indus-
