Fructose and Fructolysis 1 57 



If either glucose -alone or fructose alone is used as substrate, the 

 rate of lactic acid production is the same (Mann and Lutwak-Mann, 

 1948), but if washed spermatozoa are made to act on a 1 : 1 mixture 

 of glucose and fructose, then the rate of fructose utihzation becomes 

 much less than 50% (Mann, \95\b). This 'sparing effect' of glucose 

 on the utilization of fructose is probably due to the competitive 

 inhibition of sperm hexokinase. Slein, Cori and Cori (1950) have 

 shown that when brain or yeast hexokinase acts upon ATP and on 

 an equimolar mixture of glucose and fructose, the aldosugar is phos- 

 phorylated much more rapidly than the ketosugar. Under natural 

 conditions only fructose is present in whole semen, but not glucose, 

 so that the possibility of the latter interfering with fructolysis does 

 not arise. But there is preferential utilization of glucose in the acces- 

 sory glands, directly responsible for the accumulation of fructose, 

 and this may be due to a stronger affinity of hexokinase for glucose 

 than for fructose. A competition for hexokinase, between glucose 

 and fructose, is also consistent with the observation that in bull 

 semen incubated with an egg-yolk-diluent, the initial rate of fructo- 

 lysis is temporarily retarded (Vantienhoven, Salisbury, VanDemark 

 and Hansen, 1952), as is also the case in semen incubated with cow 

 follicular fluid (Lutwak-Mann, 1954); both egg-yolk and follicular 

 fluid contain glucose. 



The phosphohexose formed from fructose as a result of hexo- 

 kinase activity is 6-phosphofructofuranose; in the case of glucose 

 the product is 6-phosphoglucopyranose. The latter, however, is 

 readily converted to 6-phosphofructose by phosphohexose iso- 

 merase, and from this stage onwards, the enzymic degradation of 

 glucose and fructose is identical. The chain of events which in whole 

 semen leads from fructose to lactic acid, is diagrammatically depicted 

 in Fig. 15. In the normal course of fructolysis, 6-phosphofructose is 

 phosphorylated by ATP in a reaction catalysed by phosphofruc- 

 tokinase, to yield 1 : 6-diphosphofructose and ADP; disphospho- 

 fructose is next split by zymohexase into two molecules of phospho- 

 triose. Like the action of phosphofructokinase, that of zymohexase 

 was demonstrated directly in spermatozoa (Mann, 1945^). 



The subsequent steps in sperm fructolysis are analogous to the 

 corresponding phases in muscle glycogenolysis and blood glucolysis, 

 and involve the participation of cozymase (diphosphopyridine 



