Pyrophosphokinase , other

Biosynthesis of pyrophosphate (5) from pyrimidine phosphate (47) and thia2ole phosphate (48) depends on the activity of five en2ymes, four of them kinases (87). In yeasts and many other organisms, including humans, pyrophosphate (5) can be obtained from exogenous thiamine in a single step cataly2ed by thiamine pyrophosphokinase (88). 

Thiamine is absorbed by a pathway that is saturable at concentrations of 0.5-1.0 jumol/L. Oral doses in excess of 10 mg do not significantly increase blood or urine concentrations of vitamin Bi. In the human, absorption occurs predominantly in the jejunum and ileum. Some ferns, shellfish, fish, and species of bacteria contain thiami-nase, which cleaves the pyrimidine ring from the thiazole ring. This enzyme causes thiamine deficiency in cattle. In plasma, thiamine is transported bound to albumin and, to a small extent, other proteins. TPP is synthesized in the liver by thiamine pyrophosphokinase. 

Ribose phosphate pyrophosphokinase from tumor cells is inhibited by excess ATP, ADP, CTP, and other triphosphates, all of which are competitive with respect to MgATP inhibition by MgGTP is not competitive (40). The erythrocyte enzyme is inhibited by ADP, and this is relieved by increased phosphate increased ATP has no effect. GDP also inhibits, but this is not affected by phosphate 2,3-diphosphoglycerate is also a potent... 

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