122 The Biochemistry of Semen 



borate buffer (0-08m) to the extent of 73 and 85% respectively. On 

 the basis of tests with numerous phosphorylated compounds, it 

 may be safely concluded that 5-nucleotidase is an enzyme which acts 

 specifically on substrates containing the ribose-5-phosphate moiety. 

 The purified enzyme splits rapidly ribose-5-phosphate but not 

 ribose-3-phosphate; it is active towards adenosine-5 '-phosphate, 

 inosine- 5 '-phosphate, uridine-5'-phosphate and cytidine-5'-phos- 

 phate but inactive towards both adenosine-3 '-phosphate and 

 adenosine-2'-phosphate. It also dephosphorylates nicotinamide 

 mononucleotide (nicotinamide ribose-5'-phosphate); this inciden- 

 tally explains an early observation of ours, that bull seminal plasma 

 decomposes cozymase, with slow liberation of inorganic phosphate. 



Pyrophosphatase 



Bull seminal plasma contains an enzyme which hydrolyses in- 

 organic pyrophosphate to orthophosphate but differs from the 

 pyrophosphatase of yeast. Seminal pyrophosphatase can exert its 

 maximal activity in the absence of magnesium ions and is not 

 inhibited by increased substrate concentration; it has a sharp 

 optimum at pH 8-6 (Heppel and Hilmoe, \95\b), 



Enzymic hydrolysis of adenosine triphosphate 



In addition to adenosine-triphosphatase (ATP-ase) in the sperma- 

 tozoa there are also ATP-splitting enzymes in the seminal plasma 

 (Mann, 1945o; MacLeod and Summerson, 1946). When adenosine 

 triphosphate is acted upon by bull or human seminal plasma, all 

 three phosphate groups are set free as orthophosphate. The mechan- 

 ism of this reaction has been investigated by Heppel and Hilmoe 

 (1953) who by fractionation procedures obtained three distinct 

 ATP-ases, none of them, however, completely free from 5-nucleo- 

 tidase. 



One of the enzymes, named the 'pyrophosphate-forming ATP- 

 ase', catalyses the reaction 



Adenosine triphosphate +H2O — >- 



Pyrophosphate+Adenosine-5'-phosphate 



It is relatively heat-stable, has a pH optimum at 8 -4-8 -6 and 

 requires neither calcium nor magnesium ions for activation. The 



