Prolidase deficiency is a rare autosomal recessive disorder characterized by iminodipeptiduria, severe
skin ulcers,
recurrent infections, and
mental retardation. The
enzyme prolidase hydrolyzes
dipeptides containing C-terminal
proline or
hydroxyproline. We investigated the metabolic abnormality caused by
prolidase deficiency in human cultured skin fibroblasts. These studies were undertaken to test biochemical hypotheses regarding the metabolic origins of the skin lesion occurring in this disease. Our results indicate that
prolidase plays a major role in the recycling of
dipeptide-bound
proline. Control fibroblasts were able to use iminodipeptides in lieu of
proline to sustain normal growth, whereas cells homozygous for the
prolidase deficiency mutation were not.
Proline derived from iminodipeptides diluted incorporation of radiolabeled extracellular
proline into cellular
protein in normal cells but not in mutant cells. Substitution of a
prolidase-free medium for FCS did not affect the growth rate of control cell lines but increased the doubling time of
prolidase-deficient cells by 19% (28% in the presence of iminodipeptides). Iminodipeptides added to control and mutant cells maintained in serum-free medium showed no adverse effects on
protein synthesis. These results are consistent with a mechanism of biochemical pathology in which
proline deprivation caused by the
enzyme deficit is a primary cause of damage to skin cells.
Prolidase regulation by product and substrate was studied. A 44% decrease in activity was observed in fibroblasts grown for 3 wk in
proline-containing medium relative to
proline-free medium. However, cells grown in medium in which iminodipeptides replaced
proline showed no significant difference in
prolidase activity.