Dithiothreitol decreases in vitro activity of ADP-glucose pyrophosphorylase from leaves of apple (Malus domestica Borkh.) and many other plant species

Inclusion of dithiothreitol (DTT) in the extraction buffer and pre-incubation of apple leaf ADP-glucose pyrophosphorylase (AGPase) with DTT resulted in a decrease in AGPase activity whether the assay was performed in the presence or absence of 3-phosphoglycerate (PGA). When PGA was included in the pre-incubation mixture or when pre-incubation of AGPase with PGA was followed by DTT, the latter did not cause any decrease in AGPase activity. However, once AGPase was decreased by DTT, subsequent incubation of the enzyme with PGA did not reverse the decrease. Pre-incubation of AGPase from leaves of Arabidopsis thaliana, sorghum, soybean, tobacco, spinach, wheat, barley, tomato and potato, and tubers of potato with DTT, generally caused a decrease in AGPase activity when assayed in the presence of PGA. When assayed in the absence of PGA, however, a diverse response of AGPase was observed among species to pre-incubation with DTT. The activity of AGPase from potato tubers was increased by DTT; the activity of AGPase from both potato and tomato leaves was not affected by DTT; the activity of AGPase from leaves of other species was decreased by DTT. It is concluded that DTT decreases in vitro activity of AGPase from leaves of apple and many other plant species such that DTT should not be routinely included in the extraction or assay mixture of leaf AGPase.