FREE AMINO ACIDS IN ANIMAL TISSUE 343 
for single representative diabetic and control animals are shown for muscle, kidney 
and brain in Figs. 307-312. Recently a column-chromatographic study was reported 
of the content of 16 constituents in extracts of livers of four Wistar rats, made diabetic 
by the intravenous injection of 50 mg/kg of alloxan, and two controls®’. There were 
found to be decreases in the contents of aspartate, alanine, serine and threonine in 
the livers of the diabetic animals so that the ratios of the control to diabetic values 
were as follows: aspartate, 5.6; alanine, 3.8; serine, 5.2; threonine, 6.5. Such changes 
were not found in our experiments. Results are shown for the livers of four pairs 
of rats in Figs. 313-320. The contents of free amino acids were not altered in any 
detectable manner in the livers of the diabetic animals. There appeared to be a 
random fluctuation of taurine content, which has been mentioned before. The above 
results show that the severe deficiency in carbohydrate metabolism accompanying 
diabetes and the secondary metabolic disturbances which result need not be accom- 
panied by marked changes in steady-state concentrations of the free amino acids, 
although the concentrations of alanine and aspartic and glutamic acids, amino 
acids which can be formed from intermediates of carbohydrate metabolism, and 
glutamine and GABA, which are made from glutamic acid, could be regulated in 
part by availability of carbon from carbohydrate intermediates. Indeed, in extracts 
of brains of rats in hypoglycemic coma produced by insulin there were found de- 
creases in alanine and glutamic acid contents and an increase in aspartic acid®—7!, 
the changes in the latter two amino acids being isomolar. These results could be 
explained on the basis of decreased availability of glucose to brain tissue because 
of the hypoglycemia. There would be less pyruvate available for alanine formation 
and the lowered level of acetyl coenzyme A could result in decreasing the rate of 
condensation of acetyl-CoA with oxalacetate to form citrate, thus allowing the trans- 
amination of glutamic acid with oxalacetate, forming aspartate and a-ketoglutarate, 
to proceed at a more rapid rate. Fluoroacetate, which blocks the tricarboxylic acid 
cycle prior to the a-ketoglutarate oxidase step, produced a reduction in the contents 
of both glutamic acid and aspartic acids in the brains of rats7!. Although the rates of 
incorporation of amino acids into protein may be decreased in tissues of diabetic ani- 
mals*, 73, this would not appear to be related to changes in the total extractable pool. 
Effect of thyroidectomy and injection of thyroxin. It has long been known that the 
thyroid hormone plays an important role in the regulation of protein metabolism, 
an absence of the hormone resulting in retardation of growth and development in 
immature animals and disturbances in nitrogen metabolism as well as other aspects 
of tissue function in mature animals” (and see ref. 75 for further pertinent references). 
Recent work with cell-free homogenates of rat liver showed that daily pretreatment 
of rats with roo wg of sodium L-thyroxin for an average period of ro days resulted 
in an increased 77 vitvo incorporation of amino acids over that found in homogenates 
of livers of normal controls and that there was a reduction of this rate in livers of 
thyroidectomized animals’. The results suggest the possibility that thyroxin may 
play a role in linking oxidative phosphorylation to protein synthetic processes. 
We have made a paper-chromatographic study of the tissues of thyroid-injected 
and thyroidectomized rats to determine whether the physiological changes produced 
by these treatments would be reflected by any alterations in the patterns of the ex- 
tractable amino acids. The injection of tog of thyroxin daily for 15 days into 
References p. 348/349 
