The metabolism of fructose polymers in plants. 4. β-fructofuranosidases of tubers of Helianthus tuberosus L

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Abstract
Three [beta]-fructofurano-sidases were separated from a protein preparation of Jerusalem artichoke tubers by chromatography on diethylaminoethylcellulose. One of these was characterized as an invertase as it hydrolysed sucrose. The other 2 (hydrolases A and B) were similar to one another in their overall action: they were practically inactive against sucrose, but attacked a series of straight-chain oligosaccharides [(lF-fructosyl)n-sucrose, prepared from artichoke tubers] by hydrolysing terminal fructose residues from the chains; they were practically inactive against bacterial levan [[beta]-(2[forward arrow]6[image])-linkages] and against irisin [mixed [beta]-(2>1[image])-linkages and (2[forward arrow]6[image])-linkages]. Hydrolases A and B differed in their physical characteristics, and in their susceptibility to inhibition by sucrose, B being more susceptible than A. They showed small, but definite, differences in their action. Comparison of the action of hydrolases A and B on 2 series of substrates containing [beta]-(2[forward arrow]1[image])-linkages showed that the rate was largely determined by D.P., being negligible for D.P. 2(sucrose, inulobiose) and increasing over the range D.P.3 to about D.P. 6: there was an optimum chain length (undetermined), since oligosaccharides of D.P. 5-8 (the highest tested) were hydrolysed about twice as fast as was inulin (average D.P. about 30); it was shown that each enzyme attacks substrates of high and low D. P. Comparison of the rate of action of hydrolase B on various substrates containing chains of [beta]-(2[forward arrow]1[image])-linked fructose residues with various groups attached to the "reducing" end showed that not all glycosyl residues contribute equally to the effective D.P. of a substrate. Several sugars, notably sucrose, and all containing fructosyl residues, inhibited the activity of hydrolase B against inulin; free fructose was without effect. The nature of sucrose inhibition was noncompetitive. The physiological significance of these enzymes is discussed. Their probable involvement in the mobilization of fructosans when artichoke tubers sprout has been proposed in an earlier paper. It is shown here that the properties of the hydrolases cannot alone account for the distribution of fructose among various oligosaccharide fractions nor for the depolymerization which occurs when artichoke tubers are stored at low temperature, and so the method of control of these processes is still unexplained.