Protein Constituents and Enzymes of Seminal Plasma 121 



Alkaline phosphatase has an optimum at about pH 9, and is 

 capable of hydrolysing among others, 1-phosphofructose, 6-phos- 

 phofructose and 1 : 6-diphosphofructose. Our researches indicate 

 that this activity may represent an essential step in the process of 

 fructose formation and secretion by the accessory organs (Mann 

 and Lutwak-Mann, 1951«, b) (see also p. 150). Ram seminal plasma 

 is particularly rich in alkaline phosphatase which, though it acts 

 optimally at pH 9, also shows appreciable activity towards phos- 

 phohexoses at pH 7. This can be seen from Table 17 which gives 

 rates of dephosphorylation for various compounds. In the case of 

 6-phosphofructose and 1 : 6-diphosphofructose, some glucose is 

 formed in addition to fructose, owing to the presence of phospho- 

 hexose isomerase in the seminal plasma, as a result of which a part 

 of 6-phosphofructose is converted to 6-phosphoglucose before 

 dephosphorylation. Supporting evidence for the conclusion that 

 monophosphofructose rather than diphosphofructose, is the sub- 

 strate immediately responsible for the liberation of seminal fructose, 

 has been provided by Bouchilloux and Menager (1952) who found 

 that the semen of both ram and bull contains two phosphomono- 

 esterases, with pH optima at 9-4 and 4-8, respectively, but that it 

 lacks a specific fructosediphosphatase. 



5-Nucleotidase 



This enzyme was discovered in human semen by Reis (1937, 

 1938, 1940) and shown to dephosphorylate muscle adenylic acid 

 (adenosine-5 '-phosphoric acid) and inosinic acid (inosine-5'-phos- 

 phoric acid) but not adenosine-3 '-phosphoric acid or adenosine 

 triphosphoric acid. Bull seminal plasma is particularly rich in 5- 

 nucleotidase, the seminal vesicles being the main source of the 

 enzyme (Mann, 1945a, 1947). Bull seminal plasma or the vesicular 

 secretion itself, act several hundred times more efficiently on muscle 

 adenylic acid than on /S-phosphoglycerol; from 160 ^g. P added as 

 sodium adenylate to 0001 ml. bull seminal plasma, up to 140 /^g. 

 P are liberated as orthophosphate during 1 hour's incubation at 37°. 



The 5-nucleotidase of bull seminal plasma has been purified about 

 fifty-fold by Heppel and Hilmoe (195 la). The purified enzyme 

 has a pH optimum at 8-5, and its activity is enhanced by the addi- 

 tion of magnesium ions but inhibited by fluoride (00 1m) and by 



