The reduced initial product (PVX-PLP) of the reaction of pyridoxyl 5′-phosphate (PLP) and particles of potato virus X (PVX) contains about 1 (0.6 to 1.2) molecules of PLP per protein subunit and is infective. Hydrolysates of the protein contain -ε-pyridoxyl-lysine. PVX-PLP reacts with chlorogenoquinone to 1/4 of the extent of PVX; similarly, PVX which has reacted with chlorogenoquinone (PVX-Q) binds only 1/3 to 1/5 as much PLP as does PVX. PVX-PLP contains two types of fluorescent subunit which can be separated by electrophoresis in SDS-acrylamide gels: one of these is fluorescent and is not degraded by brief exposure to trypsin, whereas the other is degraded to a smaller form which is also fluorescent. Tryptic digests of the protein from PVX-PLP contain at least two fluorescent peptides. It is argued that PLP reacts with two lysine ε-amino groups of PVX, one of which also reacts with chlorogenoquinone, and the other of which is recognized by trypsin.

Protein isolated from PVX reacts with up to 6 molecules of PLP. The conformation of the subunits in the intact virus apparently makes many ε-amino groups inaccessible to PLP.


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