Present in proteins of animal origin. Absent in zein and present in very small amount in such a vegetable protein as gliadin. It is the only amino-acid with a straight chain which does not form glucose.

Dakin1 gave lysin to a phlorhizinized dog, but found neither extra sugar nor an increase in the β-oxybutyric acid excretion in the urine. This is explained by Ringer2 on the ground that lysin may be converted into glutaric acid, HOOC.C3H6.COOH, which does not form glucose when administered in phlorhizin glycosuria.3 The small increase in β-oxybutyric acid elimination noted by Ringer in this experiment does not necessarily indicate that this substance is an intermediary metabolism product.

The oxidation to COOH of the -C, to which an NH2 group is attached, followed by β-oxidation, would yield aspartic acid, provided the NH2 in the ε-position remained untouched. Such a course of metabolism would cause lysin to yield glucose in abundance and is therefore excluded.

Bacteria in intestinal putrefaction convert lysin into cadaverin, NH2- C5H40 - NH2, through simple C02 cleavage. In severe cases of cystinuria the diamines cadaverin and putresin (see p. 204) appear in the urine and this constitutes diaminuria.4