ISOLATION OF A Q-ENZYME WITH MR 103000 FROM POTATO-TUBERS

Q-Enzyme (EC 2.4.1.18) from potato (Solanum tuberosum cv Bintje) has been purified and partly characterized. The purification procedure, which includes adsorption to DEAE cellulose as well as precipitations with poly(ethylene glycol) and (NH4)2SO4, takes no more than 4 hr. The enzyme from tubers stored at 5-10-degrees was purified 80-100 times and was, after further chromatographic steps, electrophoretically pure. These preparations consisted of mono and dimeric forms of the Q-enzyme with monomeric M(r)s ranging from 86 000-103 000 whereas in preparations from fresh tubers (grown in greenhouse) 103 000 protein predominated. DEAE cellulose chromatography, omega-aminobutyl agarose chromatography and gel filtration of the preparation gave uniform peaks containing all the Q-enzyme forms. Incubation of the purified enzyme at 25-degrees for one week caused partial hydrolysis of the 103 000 component down to 86 000 but without loss of activity. The K(m) value of the enzyme from stored tubers was determined to be 0.02 mg ml-1 and kcat was on the order of 1 000 sec-1 using potato amylose as the substrate. Substrate inhibition was observed above 0.25 mg ml-1 amylose. The M(r) of the enzyme obtained in this work is higher than in other studies and possible reasons for this discrepancy are discussed.