SOLUBILIZATION AND PARTIAL CHARACTERIZATION OF PECTIN METHYLTRANSFERASE FROM FLAX CELLS

Pectin methyltransferase complex was easily solubilized from the endom embranes of flax cells (Linum usitatiasimum L.), when using Triton X-1 00 detergent. The apparent KM for the methyl donor S-adenosylmethionin e (SAM) was about 0.5 mu M, when using polygalacturonic acid as an exo genous methyl-acceptor. In the presence of saturating concentration of SAM, the apparent K-M of flax pectin methyltransferase complex for po lygalacturonic acid was 0.5-0.7 mg ml(-1) or 2.5 to 3.5 mM when consid ering the concentration of galacturonic acid monomers, Optimum activit y in vitro was observed at pH 7.1. The pectin methyltransferase activi ty was borne by several polypeptides, whose molecular relative mass va ried from very low (Mr less than or equal to 5000) to very high (Mr gr eater than or equal to 150,000) values. Several isoforms were detected , from acidic to very basic pH, with two main forms: a basic form (pI 8-8.5) of high apparent molecular mass (greater than or equal to 150,0 00) and a neutral form (pI 6.5-7.5) mainly found with intermediate mol ecular relative mass proteins (80-120,000). The neutral form presented a higher affinity (K-Mapp 0.05 mu M) than the basic form (K-Mapp 0.15 mu M) for the donor of methyl SAM.