Mechanism of Inhibition of Rice Bran Lipase by Polyphenols: A Case Study with Chlorogenic Acid and Caffeic Acid

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 µMfor CGA and CA, respectively. Fluorescence emission measurements indicated a decrease in the fluorescence
emission intensity and a red shift in the emissionmaximumas these ligands concentrations are increased, indicating
theminorchangesin the tryptophanenvironmentandthe effect ofbindingthat is strongerin the case ofCAcompared
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 ofCGAor 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 ◦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.