72 STUDIES ON ALKALINE PHOSPHATASE 



On the other hand, it may be that the alkaline phosphatase is 

 intracellular, and, in fact, acting as a phosphokinase in relation 

 to secretion; if this is so, the enzyme found on the borders of 

 cells must have the function of transferring phosphate molecules 

 from a precursor such as adenosine triphosphate to substances 

 such as glucose as soon as the latter penetrate into the cell, thus 

 preventing back diffusion. Much more study is needed of the 

 details of organization of this system before any further conclu- 

 sions may be drawn. 



Secretion by Contractile Proteins 



In the meantime a quite different suggestion for a mechanism 

 of secretion has been made by R. J. Goldacre. He has suggested 

 that secretion is carried out by the folding and unfolding of 

 protein molecules. His basic concept is that, when a protein 

 molecule is extended or unfolded, groups are exposed which are 

 able to adsorb specific molecules. Then, if later the protein 

 molecule becomes folded, the adsorbing groups may saturate 

 their affinities intramolecularly, so that the adsorbed molecules 

 become desorbed. Impressive evidence that this may occur has 

 been obtained by studying the adsorption of dyes on proteins. 

 For example, globular serum albumin adsorbs comparatively 

 small amounts of dyes, whereas when spread in a monolayer it 

 adsorbs large amounts. Similar observations have been made on 

 tobacco mosaic virus and other proteins. Consequently, we may 

 consider it adequately demonstrated that the folding of proteins 

 could be one of the basic processes in secretion. Is this process 

 one which can be reconciled with experimental studies of se- 

 cretion? 



Now Osterhout has pointed out that a great deal of the data on 

 the secretion of ions is not readily compatible with the idea that 

 there is a rapid movement of ions through cell membranes by 

 thermal diffusion. The evidence is more in favour of a one-way 

 mechanism in which, for example, an ion such as potassium in 

 the environment combines with a molecule of the cell membrane, 

 following which the complex so formed diffuses across the mem- 

 brane and discharges the ion into the interior of the cell. In 

 some peculiar way no back diffusion occurs. With potassium, for 

 example, as the external potassium concentration is raised the 

 rate of transport of potassium decreases relative to the external 



