A simple, rapid extraction and assay procedure for the NAD+-dependent sorbitol dehydrogenase (SDH) in peach

Sorbitol is the major photosynthetic product in peach. In sink tissues, sorbitol is converted to fructose via the NAD+-dependent sorbitol dehydrogenase (SDH). A new procedure is described that allows rapid, simple quantification of SDH activity in growing tissues. The procedure uses only 0.01 to 2.5 grams of fresh tissue per sample, such that a single shoot tip, a single root tip, or about 5 g of fruit flesh can be assayed for SDH activity. Storage of samples at 4 or -20 C° over night resulted in significant loss of enzyme activity. Thus, freshly harvested tissues were ground with sand in buffer at 2°C in a mortar and pestle, and the homogenate centrifuged @ 3000 ´ g to remove particulate matter and sand. The supernatant was desalted on a Sephadex G-25 column, and the eluent assayed for SDH activity immediately. Activity was determined by measuring the production of NADH per min in the assay mixture using a spectrophotometer (@340 nm). Tris buffer at pH 9.0 was the best for extraction of peach SDH. Activity of SDH was strongly inhibited by dithiothreitol (DTT) in the extraction mixture and by DTT, L-cystein, or SDI-158 in the assay mixture, similar to results reported for SDH from mammalian tissues. Peach SDH has a Km of 37.7 mm for sorbitol and a pH optimum of 9.5, similar to those reported for apple SDH. Unlike older protocols for SDH activity in plant tissues, the new procedure features reduced sample size (1/10 to 1/100th that previously used), smaller volumes of buffer, fewer buffer ingredients, greatly reduced time for sample preparation, yet comparable or higher values of SDH specific activity. Following the same procedure SDH activity was also measured in Prunus fremontii, Prunus ilicifolia, and Marianna 2624 plum (P. munsoniana × P. cerasifera).