Changes in potato tuber invertase and its endogenous inhibitor after slicing, including a study of assay methods

The increase in the invertase activity of extracts from freshly cut potato (Solanum tuberosum L.) by "foaming," caused by selective denaturation of an endogenous invertase inhibitor, did not occur in extracts made from thin disks 2 days after slicing. Rather, foaming such extracts decreased invertase activity. Apparently, the inhibitor disappeared after slicing, and the enzyme became more labile to foaming. Such disappearance of inhibitor could account for up to 15% of the dramatic increase in total invertase activity that had occurred within 2 days after slicing. The difference between extracts from 0-day and 2-day slices was mainly in the first of two peaks of invertase activity eluted from diethylaminoethyl-cellulose columns. This peak was increased by foaming 0-day extracts, but even when foamed was much smaller than in 2-day extracts. The apparent loss in inhibitor was not caused by a decreasing susceptibility of the enzyme to the inhibitor. Both the increase in total invertase activity and the apparent loss of inhibitor after slicing were partially blocked by actinomycin D and completely blocked by cycloheximide.The presence of the inhibitor can lead to serious errors in the usual whole disk method of assay for invertase in slices. Ethyl acetate treatment reduces the solubility of the enzyme but does not inactivate the inhibitor.