Diphosphoric acid triesters

P-Nucleoside P -steroyl diphosphoric acids via diphosphoric acid triesters... 

The main purpose of this section is to introduce derivatives of phosphoric acid. The acid chloride derivative is C1P0(0H)2, chlorophosphoric acid, and the amide derivative is phosphoramic acid, HgN-POCOHlg. These acid derivatives will not be discussed because they will not be used in this book. There are three ester derivatives of phosphoric acid a monoester 203, a diester 204, and a triester 205. There are also derivatives of pyrophosphoric acid (diphosphoric acid, 206), which include various esters. 

Phosphatases are numerous and important enzymes (see also Chapt. 2). They are classified as phosphoric monoester hydrolases (phosphatases, EC 3.1.3), phosphoric diester hydrolases (phosphodiesterases, EC 3.1.4), triphosphoric monoester hydrolases (EC 3.1.5), diphosphoric monoester hydrolases (pyrophosphatases, EC 3.1.7), and phosphoric triester hydrolases (EC 3.1.8) [21] [63]. Most of these enzymes have a narrow substrate specificity restricted to endogenous compounds. However, some of these enzymes are active toward xenobiotic organophosphorus compounds, e.g., alkaline phosphatase (EC 3.1.3.1), acid phosphatase (EC 3.1.3.2), aryldialkylphosphatase (para-oxonase (PON1), EC 3.1.8.1) and diisopropyl-fluorophosphatase (tabunase, somanase, EC 3.1.8.2) [64 - 70]. However, such a classification is far from definitive and will evolve with further biochemical findings. Thus, a good correlation has been found in human blood samples between somanase and sarinase activities on the one hand, and paraoxonase (PON1) type Q isozyme concentrations on the other [71]. 

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