A textbook of biochemistry for students of medicine and science / by A.T. Cameron.

  • Cameron, A. T. (Alexander Thomas), 1882-1947
Date:
1933
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Licence: In copyright

Credit: A textbook of biochemistry for students of medicine and science / by A.T. Cameron. Source: Wellcome Collection.

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    bringing about various intermediate stages (compare the action of the oxides of nitrogen in the formation of sulphuric acid), or they may act merely by physical means, through bringing molecules of the reacting substances into closer juxtaposition (as with the adsorption of hydrogen and oxygen on the large surface of spongy platinum). While the great majority of catalysts accelerate reactions, some, termed negative catalysts, lengthen the time of reaction or suppress the action altogether. When a stick of phosphorus is exposed to the atmosphere it slowly oxidises. The presence of a trace of ether vapour stops this oxidation. While traces of hydrogen sulphide markedly increase the rapidity of oxidation of stannous chloride exposed to air, manganese and chromium salts and certain organic compounds such as mannitol and aniline greatly reduce the speed of oxidation. It has been suggested that a “ negative catalyst ” produces its results indirectly, by suppress- ing the action of some true catalyst. Biochemical Catalysis The preparation of alcohol and carbon dioxide from sugar by yeast is one of the oldest known chemical reactions brought about by a living organism. Similar long known reactions are the bacterial transformation of lactose to lactic acid which occurs in the souring of milk, and of wine (a solution of alcohol) into vinegar (a dilute solution of acetic acid), and the transformation of starch into glucose by germinating barley. The role of the living organism in producing these reactions was first definitely recognised by Pasteur. In 1832 Payen and Persoz prepared an extract from barley malt which converted starch into sugar just as do strong acids, although no living cells were present. This extract they called diastase (whence, until quite recently, French writers have called all enzymes diastases). They made the extract by macerating the germinating barley with water, and adding to the clear solution alcohol, which precipitated the diastase as a white powder, soluble in water.