Factors affecting the digestibility of raw and gelatinized potato starches

The enzymatic digestibilities of raw and gelatinized starches in various potato starches, as well as sweet potato, cassava, and yam starches, were estimated, along with other starch properties, such as the phosphorus content, median granule size, and rapid visco analyzer (RVA) pasting properties. Furthermore, correlation coefficients were calculated between the hydrolysis rates (HR) by amylase and other starch quality parameters. A larger granule size was closely associated with a lower HR in raw starch, while the HR in gelatinized starch did not correlate with the median granule size. An increase in phosphorus content resulted in a definitely lower HR in raw starch and tended to decrease the HR in gelatinized starch for the composite of potato and other starches. In contrast, no correlation coefficients of the phosphorus content with the HRs in raw and gelatinized starches were observed within potato starches. Starches with higher peak viscosity and breakdown showed a lower HR in raw starch, while few or no effects of these RVA parameters on the HR in gelatinized starch were observed for the composite of potato and other starches or among potato starches, respectively.

Mechanism of the Decline in Vacuolar H -ATPase Activity in Mung Bean Hypocotyls during Chilling

The mechanism responsible for the decrease in the activity of vacuolar H(+) -ATPase during chilling was investigated in seedlings of mung bean (Vigna radiata). After chilling at 0 degrees C for 3 d, the activity of vacuolar H(+) -ATPase, calculated on the basis of membrane protein, decreased to 47% of the original value. Of the nine subunits of the ATPase, the specific contents of at least six subunits, of 68, 57, 44, 38, 37, and 32 kD, decreased in vacuolar membranes after chilling, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These subunits were released by treatment with chaotropic anions such as thiocyanate. The level of the 16-kD subunit did not change. Immunoblot analyses showed the decrease in the levels of the subunits of 68, 57, and 32 kD. Furthermore, the specific activity of the ATPase purified from chilled hypocotyls was two-thirds of that of the enzyme from nonchilled seedlings, and the enzyme from chilled tissue retained only a small amount of the 32-kD subunit. These results suggest that a selective release of the peripheral subunits of the ATPase from the membrane and a partial degradation of the ATPase complex may occur in vivo during chilling.

Comparison of Temperature Dependency of Tonoplast Proton Translocation between Plants Sensitive and Insensitive to Chilling

Proton transport activities in isolated tonoplast vesicles were measured as quenching of fluorescence of acridine orange. A marked difference in the temperature dependency of two types of tonoplast proton transports, i.e. ATP- and pyrophosphate-driven, was observed between two leguminous plants sensitive (mung bean, Vigna radiata [L.] Wilczek) and insensitive (pea, Pisum sativum L.) to chilling. In tonoplast vesicles isolated from hypcotyls of mung bean seedlings that were germinated for 3.5 days at 26 degrees C in the dark, the total amount of fluorescence quenching at the steady state in both types of proton pumps, as a measurement of the inside-acidic pH gradient across the membrane vesicles, was markedly suppressed under temperatures below 10 degrees C. In tonoplast vesicles isolated from epicotyls of pea seedlings, which were germinated for 7 days at 18 degrees to 23 degrees C in the dark, no suppression occurred in the formations of the pH gradient in either type of proton pump, even at 0 degrees C. The cause of the low temperature-induced suppression of the proton pumps in mung bean tonoplasts seems to be not an increased permeability of the membrane vesicles to protons or accompanying anions and cations, but instead a marked inhibition in the catalytic activity of both enzymes under low temperatures.

Subunit composition of vacuolar membrane H(+)-ATPase from mung bean

The vacuolar H(+)-ATPase from mung bean hypocotyls was solubilized from the membrane with lysophosphatidycholine and purified by QAE-Toyopearl column chromatography. The purified ATPase was active only in the presence of exogenous phospholipid and was inhibited by nitrate, dicyclohexyl carbodiimide and Triton X-100, but not by vanadate or azide. Dodecyl sulfate/polyacrylamide gel electrophoresis of the purified ATPase yielded ten polypeptides of molecular masses of 68 kDa, 57 kDa, 44 kDa, 43 kDa, 38 kDa, 37 kDa 32 kDa, 16 kDa, 13 kDa and 12 kDa. All polypeptides remained in the peak activity fraction after glycerol density gradient centrifugation. Nine of them, excluding the 43-kDa polypeptide, comigrated in a polyacrylamide gradient gel in the presence of 0.1% Triton X-100. The 16-kDa polypeptide could be labeled with [14C]dicyclohexylcarbodiimide. The amino-terminal amino acid sequence of the isolated 68-kDa polypeptide generally agreed with that deduced from the cDNA for the carrot 69-kDa subunit [Zimniak, L., Dittrich, P., Gogarten, J. P., Kibak, H. & Taiz, L. (1988) J. Biol. Chem. 263, 9102-9112]. Thus, mung bean vacuolar H(+)-ATPase seems to consist of nine distinct subunits.