Pyrimidine Phosphorylases: Pentosyltransferases that catalyze the reaction between a pyrimidine nucleoside and orthophosphate to form a free pyrimidine and ribose-5-phosphate.Phosphorylases: A class of glucosyltransferases that catalyzes the degradation of storage polysaccharides, such as glucose polymers, by phosphorolysis in animals (GLYCOGEN PHOSPHORYLASE) and in plants (STARCH PHOSPHORYLASE).Phosphorylase Kinase: An enzyme that catalyzes the conversion of ATP and PHOSPHORYLASE B to ADP and PHOSPHORYLASE A.Phosphorylase b: The inactive form of GLYCOGEN PHOSPHORYLASE that is converted to the active form PHOSPHORYLASE A via phosphorylation by PHOSPHORYLASE KINASE and ATP.Phosphorylase a: The active form of GLYCOGEN PHOSPHORYLASE that is derived from the phosphorylation of PHOSPHORYLASE B. Phosphorylase a is deactivated via hydrolysis of phosphoserine by PHOSPHORYLASE PHOSPHATASE to form PHOSPHORYLASE B.Purine-Nucleoside Phosphorylase: An enzyme that catalyzes the reaction between a purine nucleoside and orthophosphate to form a free purine plus ribose-5-phosphate. EC 2.4.2.1.Uridine Phosphorylase: An enzyme that catalyzes the transfer of ribose from uridine to orthophosphate, forming uracil and ribose 1-phosphate.Pyrimidines: A family of 6-membered heterocyclic compounds occurring in nature in a wide variety of forms. They include several nucleic acid constituents (CYTOSINE; THYMINE; and URACIL) and form the basic structure of the barbiturates.Pyrimidine Nucleotides: Pyrimidines with a RIBOSE and phosphate attached that can polymerize to form DNA and RNA.Thymidine Phosphorylase: An enzyme that catalyzes the transfer of 2-deoxy-D-ribose from THYMIDINE to orthophosphate, thereby liberating thymidine.Pyrimidine Dimers: Dimers found in DNA chains damaged by ULTRAVIOLET RAYS. They consist of two adjacent PYRIMIDINE NUCLEOTIDES, usually THYMINE nucleotides, in which the pyrimidine residues are covalently joined by a cyclobutane ring. These dimers block DNA REPLICATION.Glycogen Phosphorylase: An enzyme that catalyzes the degradation of GLYCOGEN in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. This enzyme exists in two forms: an active phosphorylated form ( PHOSPHORYLASE A) and an inactive un-phosphorylated form (PHOSPHORYLASE B). Both a and b forms of phosphorylase exist as homodimers. In mammals, the major isozymes of glycogen phosphorylase are found in muscle, liver and brain tissue.Pyrimidine Nucleosides: Pyrimidines with a RIBOSE attached that can be phosphorylated to PYRIMIDINE NUCLEOTIDES.Phosphorylase Phosphatase: An enzyme that deactivates glycogen phosphorylase a by releasing inorganic phosphate and phosphorylase b, the inactive form. EC 3.1.3.17.Polyribonucleotide Nucleotidyltransferase: An enzyme of the transferase class that catalyzes the reaction RNA(n+1) and orthophosphate to yield RNA(n) and a nucleoside diphosphate, or the reverse reaction. ADP, IDP, GDP, UDP, and CDP can act as donors in the latter case. (From Dorland, 27th ed) EC 2.7.7.8.Pentosyltransferases: Enzymes of the transferase class that catalyze the transfer of a pentose group from one compound to another.Glycogen Phosphorylase, Liver Form: An isoenzyme of GLYCOGEN PHOSPHORYLASE that catalyzes the degradation of GLYCOGEN in liver tissue. Mutation of the gene coding this enzyme on chromosome 14 is the cause of GLYCOGEN STORAGE DISEASE TYPE VI.Glycogen Phosphorylase, Muscle Form: An isoenzyme of GLYCOGEN PHOSPHORYLASE that catalyzes the degradation of GLYCOGEN in muscle. Mutation of the gene coding this enzyme is the cause of McArdle disease (GLYCOGEN STORAGE DISEASE TYPE V).Starch Phosphorylase: An enzyme of the PHOSPHORYLASES family that catalyzes the degradation of starch, a mixture of unbranched AMYLOSE and branched AMYLOPECTIN compounds. This phosphorylase from plants is the counterpart of GLYCOGEN PHOSPHORYLASE in animals that catalyzes the reaction of inorganic phosphate on the terminal alpha-1,4-glycosidic bond at the non-reducing end of glucans resulting in the release of glucose-1-phosphate.Glycogen