Oxidation of D-mannosamine (1), D-glucosamine (2), and D-galctosamine (3) by sodium N-chlorobenzenesulfonamide or chloramine-B (CAB) at 313 K is followed by a shortening of carbon chain and obeys the rate law, rate = k[CAB][sugar][HO(-)](x)(), where x is less than unity. The products are arabinonic acid, ribonic acid, and erythronic acid for 1 and 2 with smaller amounts of glyceric and hexonic acids, while lyxonic and threonic acids are predominant in the oxidation of 3 with smaller amounts of glyceric and hexonic acids. Proton inventory studies made in a H(2)O-D(2)O mixture point toward a single transition state. In the proposed mechanism the alkoxy anion (S(-)) of the hexosamine formed in a base-catalyzed reaction at C-1 carbon is subjected to an electrophilic rate-limiting attack by Cl(+) of the oxidant. The hexonic acid formed is decarboxylated with loss of ammonia to form the respective pentose, which is further converted into the corresponding pentonic acid. The breaking of the bond between C-1 and C-2 carbons in pentose yields tetronic acids. The thermodynamic parameters for sugar alkoxy anion formation and activation parameters for the rate-limiting step have been evaluated.

No abstract

Rice bran is a byproduct obtained from the rice milling industry, and to arrest lipolysis caused by lipolytic enzyme, rice bran lipase (RBL) was inactivated by inhibitors such as polyphenols. This study describes the inhibition and interaction of enzyme with chlorogenic acid (CGA) and caffeic acid (CA). The inhibition of the enzyme was competitive in nature in both CGA and CA. The inhibition constant K(i) of the reaction was found to be 1.8 and 1.5 muM for CGA and CA, respectively. Fluorescence emission measurements indicated a decrease in the fluorescence emission intensity and a red shift in the emission maximum as these ligands concentrations are increased, indicating the minor changes in the tryptophan environment and the effect of binding that is stronger in the case of CA compared to CGA with RBL. Far UV-circular dichroic data suggest that there are no significant changes in the conformation of the enzyme as a result of binding of CGA or CA. The instability of the enzyme in the presence of these polyphenols has been indicated by decrease in apparent thermal transition temperatures of the enzyme from a control value of 60 degrees C as revealed by thermal denaturation measurements. These results demonstrate that both CGA and CA are inhibitors of RBL and bind to the enzyme through both hydrogen and hydrophobic interaction in bringing about inhibition with minor structural alterations. These inactivation phenomena of polyphenols that act as inhibitors on RBL can be utilized to prevent oxidation of the rice bran oil.

Introduction:

The kinetics of inhibition of rice bran lipase (RBL) by phenylboronic acid (PBA) was studied to elucidate the nature of inhibition and the effect of the inhibitor on the structure-function of RBL. The effectiveness of an inhibitor is normally expressed by the constant K(i), which is calculated from the Lineweaver-Burk plot and found to be 1.7 mM at pH 7.4. The kinetics of inhibition by PBA was competitive, indicating the presence of serine in the active site of the enzyme. The loss of activity of RBL was concentration dependent on the inhibitor (PBA), and the inactivation followed a pseudo-first-order kinetics. Fluorescence emission measurements indicated a decrease in the fluorescence emission intensity and a red shift in the emission maximum as the inhibitor concentration was increased. The inhibition of the enzyme by PBA was also confirmed by thermal denaturation measurements, which indicated a shift in the thermal denaturation temperature of the enzyme toward lower temperatures. The far-UV-CD data suggest that there were no significant changes in the conformation of the enzyme as a result of binding of PBA. These results indicate that PBA is a potential inhibitor of RBL and binds to the enzyme in bringing about inhibition without any structural alterations.