Ilyas Beg, Faizan Ahmad, Asimul Islam
Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India.

It has been known for a long time that extract from many plant tissues posses anti-viral activity. In several cases, it is due to proteins identified as inhibitor of protein synthesis, called ribosome-inactivating proteins (RIPs). Cochinin B is an important type I RIP which possesses a broad range of potent anti-tumor activities and exhibits the N-glycosidase activity like other RIPs. Cochinin B, a novel ribosome-inactivating protein (RIP) with a molecular mass of 28 KDa, was purified from the seeds of Memordica (karela) of Cucubitaceae family. We have checked its purity using SDS PAGE which was found to be more than 95%. The far-UV CD is carried out to understand the secondary structural content of cochinin B. We found more contribution of β-sheets than α-helix in the secondary structural content of the protein at pH values 8.0, 7.5, 7.0, 4.0 and 2.0, however, we noticed a transition from β-sheets to α-helix at pH 6.0. Cochinin B at pH 7.5 has maximum [θ ] 222 . The values of [θ ] 222 and α-helix content are -10777 deg cm2 dmol ^-1 and 32%, respectively. Both [θ ] 222 and α-helix content decrease; [θ ] 222 decreases from -10777 to -5264 deg cm ² dmol -1 and α-helix decreases from 32 % to 11 % as pH decreases from 7.5 to 2.0. To check the thermal stability of cochinin B we have heated the protein from 20° to 85° C at pH values 2.0, 4.0, 6.0, 7.0, 7.5 and 8.0 to observe change in [θ ] 222 a measure of secondary structure. Each denaturation curve was analyzed for ^fD , fraction of denatured protein, as a function of temperature. The value of ^Tm , the temperature at which ^fD = 0.5, were determined from the curves. Cochinin B has nearly the same Tm at pH values 8.0, 7.5 and 7.0 which are 66.9±0.1, 67.1±0.1 and 67.0±0.1 °C respectively. Tm decreases significantly at lower pH values. So, cochinin B has maximum stability and secondary structure contents at pH 7.5, and it is stable over physiological pH. However, further experiments are required to support that cochinin B is a good candidate of antiviral activity at physiological pH and can be used as effective drug.