I CYTOPLASM 155 



the loosening of the junctions is achieved according to an entirely 

 different principle, namely by addition of elementary hydrogen 

 ihydrogenation). 



cystine bridge: CH-R-SH HS-R-CH <— — ^ CH-R-S-S-R-CH 



/ 

 methylene bridge: CH-R-CH3 CH3-R-CH i===^ CH-R-CH2-CH2-R-CH 



-2H 



TTh 



Fig. 102. Bridges dependent on r^ 



At physiological temperatures water in very small amounts not only 

 dissociates into ions according to the scheme HgO ^ H+ + OH~, but 

 also, though admittedly to still less extent, into the elements hydrogen 

 and oxygen: zHgO^ 2H2 ^ Og. These gases develop a very low 

 gas pressure, which for hydrogen we shall designate as tHg. 



If the partial pressure of hydrogen in the cytoplasm increases, the 

 -S-S- bridges tend to be hydrogenated, which causes rupture of the 

 bonds (Fig. 102). The cystine bridges can therefore absorb Hg and 

 for this reason act in the same way with respect to the partial pressure 

 of Ha as a buffer with respect to the concentration of H+ions. These 

 conditions have been investigated in particular in the case of gluta- 

 thione (G). This is a protein compound which can be split into glutamine, 

 cysteine and glycine. It represents a tripeptide chain with the three 

 amino acids mentioned as side chains. However, whether it occurs in 

 the cytoplasm as a free molecule or only as part of a much larger 

 macromolecule cannot be decided at present. In both cases glutathione 

 reacts according to the following scheme : 2 GSH ^ GS-SG + Hg. 

 Thus, when sulphydryl groups occur in the side chains of proteins 

 (Fig. 96 IV, p. 145), these can give rise to formation or dissolution of 

 cross-links. 



Methylene bridges cannot be formed with the same ease, at any 

 rate in the laboratory, where methyl groups show a very passive 

 behaviour. All the same, it is known that in the metabolism one 

 molecule of succinic acid can be formed out of two molecules of acetic 

 acid by dehydrogenation (Mothes, 1933). This succinic acid is then 

 dehydrogenated further to fumaric acid, converted into malic acid, 

 dehydrogenated to oxalo-acetic acid and finally, after decarboxylating 

 this keto-acid, converted into pyruvic acid. It thus becomes apparent 



