1 10 The Biochemistry of Semen 



accounts for as much as 91% of the dry, lipid-free mass of the head 

 nuclei. These authors, however, also find in the nuclei a charac- 

 teristic tryptophan-containing residual protein. 



Keratin-like protein of the sperm membrane 



Within the category of 'residual' sperm proteins are also certain 

 highly insoluble sulphur-rich proteins, obtained from mammalian 

 spermatozoa and derived probably from the sperm membrane. The 

 first mention of a 'sulphur-rich substance, containing more than 

 4% S in the heads of bull spermatozoa', was made in 1878 by 

 Miescher, who did not, however, investigate its origin and compo- 

 sition. Green (1940) extracted ram spermatozoa successively with 

 dilute acid and alkali, and obtained a residue containing 19-3% 

 nitrogen and 11-4% cystine, which he believed to represent the 

 sperm membrane. The possibility of a keratin-like protein present 

 in the sperm membrane is strengthened by the observations of Zittle 

 and O'Dell (1941a) on the solubilizing action of thioglycolic acid 

 and trimethylbenzylammonium hydroxide on bull spermatozoa. In 

 boar sperm, the portion which remains undissolved after prolonged 

 treatment with N-NaOH, consists of 'ghost' sperm-heads which 

 resemble in shape the sperm membranes (Thomas and Mayer, 1949). 

 A remarkable property of the sperm membrane which can be demon- 

 strated microscopically, is the extraordinary elasticity of the sperm- 

 head structure; this was convincingly demonstrated in the experi- 

 ments of Moench and Holt (1929-32), who were able to hook the 

 head of a human spermatozoon with a microsurgical needle and 

 to stretch it very considerably. 



