272 A SYMPOSIUM ON RESPIRATORY ENZYMES 



slices than with tissue breis. While slices have been used in attempts 

 to determine what a tissue actually does, breis have been useful in 

 discovering essential substances and the interrelation of reactions. 

 The disintegration of tissue permits various essential materials to be 

 more readily diluted by the suspending medium or destroyed by 

 enzymes, so that their concentration falls below the optimal and 

 their addition produces striking efiFects often not found with slices. 



The preparation of slices from some tissues, especially brain, is 

 slow and delicate work, sampling is inaccurate, and one cannot be 

 sure that the individual slices do not vary in activity. Dr. Libet and 

 I (1) have recently studied the respiration of brain suspensions and 

 we find that two types of suspension can be obtained. One type 

 behaves as disintegrated tissue; the other type, when prepared under 

 proper conditions, behaves very similarly to slices, shows a com- 

 parable respiration rate, and is a suitable and very convenient 

 preparation for the study of brain metabolism. 



The first type of suspension is obtained when brain tissue is 

 homogenized by the apparatus of Potter and Elvehjem (2) in hypo- 

 tonic medium, dilute phosphate buffer solution. Such suspensions 

 respire at a low rate which may be increased up to 65 per cent by 

 adding salt or sugar after homogenization. With such preparations 

 it is therefore necessary that the osmotic pressure be equal in the 

 control and experimental flasks when the effect of added substances 

 is tested. These hypotonic suspensions have largely lost the power to 

 utilize glucose. They show considerable effects when tissue extract 

 or substances like fumarate are added. The rate of respiration per 

 unit weight of tissue increases with increasing tissue concentration. 



The second type of suspension is obtained when the medium in 

 which the brain is homogenized contains sufficient salt, sucrose, or 

 glucose to make the osmotic pressure equal to that of serum. The 

 respiration rate of such suspensions is up to 400 per cent greater than 

 the rate of tissue homogenized in hypotonic medium. (Isotonic urea 

 behaves like hypotonic solution.) Suspensions of whole brain (con- 

 taining a large amount of white matter, which respires only slowly), 

 prepared in 0.13 M sodium chloride-0.017 M phosphate buffer solu- 

 tion, respire on the average at 71 per cent of the rate of an equal 

 tissue weight of slices of pure gray matter in the same medium. 

 Such suspensions are much less affected by the addition of tissue 

 extract, fumarate, etc., than are hypotonic suspensions, and their 

 respiration rate per unit weight is practically independent of the 

 tissue concentration. Isotonic suspensions respire with the same res- 



