Pyrimidine nucleosides from reaction of sugar phosphate with pyrimidine base derivative & enzyme having cytosine nucleoside phosphorylase activity

A method for producing a pyrimidine nucleoside compound comprises the step of: ```reacting a sugar phosphate with a pyrimidine base derivative in the presence of an enzyme having cytosine nucleoside phosphorylase activity, ```wherein the pyrimidine base derivative is represented by general formula (I): wherein: ```R1 represents an amino group (that is optionally substituted by an acyl group having an alkyl group of 1 to 3 carbon atoms or by an alkyl group of 1 to 3 carbon atoms), an alkyl group of 1 to 3 carbon atoms, or a thiol group; ```R2 represents an amino group, a thiol group, a hydroxyl group, or a hydrogen atom; ```R3 represents an alkyl group of 1 to 3 carbon atoms (that is optionally substituted by a hydroxyl group), an amino group, or a hydrogen atom; ```R4 represents a hydroxyl group or a hydrogen atom; ```with the proviso that when R1 is an amino group, R2 is a hydroxyl group, and R4 is a hydrogen atom, then R3 is neither an alkyl group of 1 to 3 carbon atoms nor a hydrogen atom. The enzyme having cytosine nucleoside phosphorylase activity may be derived from E. coli and may be provided in the form of a bacterial cell of a microorganism having the enzyme, or a processed enzyme prepared from the bacterial cell, or a culture solution of the bacterial cell. Pyrimidine nucleoside compounds represented by general formula (II): wherein: ```R5 represents an amino group or a hydrogen atom; ```R6 represents an amino group or a hydrogen atom; and ```R7 represents a hydroxyl group or a hydrogen atom, are claimed per se.