FREE AMINO ACIDS OF BLOOD AND URINE 253 
taurine for 3 days prior to exposure. This experiment is based on the fact that 
intracellular taurine is characterized by a rather low turnover rate, contrary to 
its extracellular fraction, which is negligible with regard to the amount excreted. 
In those conditions, one may assume, either that if the specific activity of excreted 
taurine after irradiation does not vary or increases, that it came at least partly 
from the intracellular compartment; or, if the specific activity of excreted taurine 
after irradiation is decreased (total radioactivity being constant), that it came 
from its precursors. 48h after the last labeled-taurine injection, the test animals 
were irradiated, receiving a single 500 r external dose of X-rays, and at the same 
time control animals were submitted to mock irradiation. Within the first 24h 
following that treatment, taurine excretion in the test animals increased by 75% 
as compared to controls, and its specific radioactivity remained unchanged. Taurine 
urinary hyperexcretion may thus be attributed to escape from the cells where this 
amino acid is highly concentrated, rather than to excessive oxidation of sulfur 
amino acids. 
It seems that, as far as taurine and BAIBA are concerned, protein malnutrition 
as Well as radiation effects share some properties, which are to increase BAIBA 
formation from DNA and to injure some sensitive mechanisms, probably enzymatic 
in nature, which sustain high taurine intracellular concentration in physiological 
conditions. 
Provisional classification of aminoaciduria in diseases 
Attractive in its simplicity, DENtT’s classification*! of abnormal types of amino- 
aciduria is based on the blood amino acid clearance concept. Pathological conditions 
are divided in three categories: (a) “overflow” aminoaciduria in which the blood 
level of the amino acid concerned is definitely raised above normal; (b) “renal” 
aminoaciduria in which case the blood level is either normal or below normal, and 
yet the urinary excretion is above normal (tubular reabsorption deficiency) ; (c) “no- 
threshold” aminoaciduria in which the condition is attributed to an extra-renal 
disturbance of the metabolism of an amino acid with a high blood clearance, and in 
which, on account of this latter circumstance, the corresponding blood level may 
stay normal or is hardly increased. 
In 1958, when preparing a general review on aminoaciduria with BiGwoop et al.', 
we strongly felt that our knowledge of blood amino acid levels and clearances was 
still insufficient to fill simply the three theoretical categories of DENT with the avail- 
able data. Today we still adhere to this opinion but owing to some recent advances 
we feel the need for a revision!* of the provisional classification published in 1958. 
Accordingly we propose a renewed but still provisional classification* which will 
distinguish two main types of abnormal aminoaciduria: (1) a common type of amino- 
aciduria, involving more or less all amino acids excreted in urine; (2) a specific type 
of aminoaciduria, involving a limited number of urinary amino acids. 
An extensive reference list is to be found in the review published by BiGwoop 
et al.*, 
* Based mainly on the views of Dr. H. Vis; see footnote page 250. 
References p. 261/262 
