BODY FLUIDS AND EFFECT OF HEREDITARY DISORDERS 217 
the liver even the reduced concentration of ASase is sufficient to enable some of the 
ASA to be further metabolized to urea. 
f-Aminotsobutyric-aciduria. In the course of routine studies of the amino acids in 
urine by paper chromatography it was observed that a number of apparently norma! 
healthy people, about one in twenty, excreted as much /-aminoisobutyric acid (BAIBA) 
as they did glycine: 4. The amount of BAIBA excreted (Fig. 16) remained constant 
for the individual, was uninfluenced by diet, and from family studies it appeared to 
be under genetic control. The frequency of incidence of “high excretors” varies with 
the racial groups and are highest in the Mongolian races and in the American In- 
dians®®: 8°. BAIBA seems to be present in most human urine samples but usually 
only in trace amounts. The blood plasma level of BAIBA is barely detectable and 
it is probably one of those substances which are not reabsorbed at all by the kidney 
tubule®®. It is thought that BAIBA arises from the breakdown of DNA via thymine” 
but there is also an alternative source arising from the degradation of valine®!. The 
most likely explanation, at present, for the high excretion of BAIBA by some people 
is that they have a partial block in its further metabolism. 
Cystathioninuria. Another interesting form of aminoaciduria has recently been dis- 
covered by Harris®® during a systematic screening of the urines of patients in a 
mental deficiency institution. This patient excreted approx. 500 mg/day of cysta- 
thionine (Fig. 16) and the output was markedly increased by feeding methionine. 
Cystathionine is a postulated intermediate in the series of reactions taking place in 
the conversion of methionine to cystine. This substance, like argininosuccinic acid 
and f-aminoisobutyric acid in the previous diseases, is probably an intracellular 
substance only and rarely reaches the extracellular fluids. Cystathionine has recently 
been found in brain tissue®?. Although no other evidence is available it seems likely 
that this condition is caused by a lack or shortage of the enzyme responsible for 
cleaving cystathionine to form cysteine and homoserine. 
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