196 R. G. WESTALL 
measurement of the amino acids and attempts are still being made currently to im- 
prove the quantitation. At the present time, the elegant method of ion-exchange 
column chromatography devised by MOORE AND STEIN in its latest partly automatic 
form’ provides the most accurate means of estimating the commonly occurring amino 
acids. Nevertheless, paper chromatography with its simple apparatus, rapid results 
and adaptability will remain a valuable technique in biological research for a long time. 
The body fluids 
Blood plasma. Studies on the amino acids present in blood plasma or serum have been 
carried out by numerous workers who have been interested in the metabolic fate 
of these substances in the body. The systemic blood which replenishes its store of 
amino acids from the gut via the portal system and the liver can be regarded, in a 
general way, as the medium from which the tissue cells, by way of the extracellular 
fluid, draw their nutriment and into which they discard the unwanted by-products. 
Since a proportion of the cellular protein is being continuously broken down and new 
protein js being synthesized it might be expected that the nutrient medium would 
have an optimum composition especially with regard to those amino acids which are 
protein constituents. In fact the amino acid composition of the plasma, in the fasting 
condition, has a fairly constant pattern which simulates approximately the amino 
acid composition of the tissue proteins. The plasma amino acid level rises after a 
protein meal® but it slowly drops back to the fasting level within approx. 4 h. Those 
amino acids which are protein constituents, and this, of course, includes those which 
are essential to our diet and which we are unable to synthesize ourselves in sufficient 
quantities, if at all, are only excreted in the urine in amounts of the order of 1-2°%, of 
the dietary intake. Glycine and histidine, which are not so essential, are found in the 
urine in larger amounts (up to 20%). Hence it follows that since in good nutritional 
status the intake of the essential amino acids exceeds the amount required for protein 
replacement, in the adult, these amino acids in excess must be used in other ways. 
That this is true is well exemplified in the two metabolic diseases phenylketonuria and 
maple-syrup-urine disease where in the first case phenylalanine and in the second 
instance the branched chain amino acids leucine, isoleucine and valine cannot be 
further metabolized at the required rate due to an insufficiency of specific enzymes 
and these amino acids build up to a bigh concentration in the body fluids. On the other 
hand, when the nutritional status is poor and the individual is in negative nitrogen 
balance there is a tendency for the plasma amino acid level to fall. For a while this 
fall in level is prevented from going too far by the utilization of endogenous supplies 
of expendable tissue protein but once this store has been used up the plasma 
amino acid level begins to fall seriously. This is the state of affairs frequently found in 
cases of kwashiorkor and other forms of protein malnutrition where the plasma amino 
acids may fall to as low as one-fifth of the normal concentration. 
A two-way paper chromatogram showing the amino acid pattern in human adult 
blood plasma is shown in Fig. 1. For this chromatogram a sample of 625 yl of ultra- 
filtered and desalted plasma was used as suggested by DENT®. A number of quantitative 
analyses, performed by various workers, are given in Table I and a composite range 
of values is given in the final column. 
Aarpt from those amino acids which are utilized directly for protein synthesis 
References p. 217/219 
