Combination of potato virus X and tobacco mosaic virus with pepsin and trypsin

Abstract

Pepsin combines with potato virus X, with casein and with serum proteins, which are substrates for its proteolytic activity, but not with tobacco mosaic virus, which is not a substrate. When this virus is turned into a substrate as a result of heat denaturation, then combination takes place. Thus, details of structure of proteins that determine their susceptibility to proteolytic activity of pepsin, also seem to determine the ability of pepsin to combine with proteins. This is not true of trypsin, which combines with all the proteins tested, ir- respective of whether they are substrates for its proteolytic activity or not. Even more trypsin combines with tobacco mosaic virus, which is not a substrate, than with potato virus X, which is a substrate. The combination of trypsin with tobacco mosaic virus could account for the reversible inhibition of infectivity of the virus. This combination protects trypsin from spontaneous inactivation at pH 7. Invertase does not combine with potato virus X, with tobacco mosaic virus whether heat denatured or not, or with casein. Trypsin and invertase adsorbed on charcoal can be set free by casein; invertase can also be extracted by tobacco mosaic virus, but not by sucrose. Therefore, the extraction of an enzyme from charcoal is clearly no indication that the substance responsible for the extraction is a substrate or that it can even combine with the enzyme.