A futile cycle, formed between two ATP-dependant gamma-glutamyl cycle enzymes, gamma-glutamyl cysteine synthetase and 5-oxoprolinase: the cause of cellular ATP depletion in nephrotic cystinosis?
Cystinosis, an inherited disease caused by a defect in the lysosomal cystine transporter (CTNS), is characterized by renal proximal tubular dysfunction. Adenosine triphosphate (ATP) depletion appears to be a key event in the pathophysiology of the disease, even though the manner in which ATP depletion occurs is still a puzzle. We present a model that explains how a futile cycle that is generated between two ATP-utilizing enzymes of the gamma-glutamyl cycle leads to ATP depletion. The enzyme gamma-glutamyl cysteine synthetase (gamma-GCS), in the absence of cysteine, forms 5-oxoproline (instead of the normal substrate, gamma-glutamyl cysteine) and the 5-oxoproline is converted into glutamate by the ATP-dependant enzyme, 5-oxoprolinase. Thus, in cysteine-limiting conditions, glutamate is cycled back into glutamate via 5-oxoproline at the cost of two ATP molecules without production of glutathione and is the cause of the decreased levels of glutathione synthesis, as well as the ATP depletion observed in these cells. The model is also compatible with the differences seen in the human patients and the mouse model of cystinosis, where renal failure is not observed.
Kumar
Akhilesh
A
Institute of Microbial Technology, Sector 39-A, Chandigarh 160 036, India.
Bachhawat
Anand Kumar
AK
D000255:Adenosine Triphosphate / Q000737:chemistry*; D026921:Amino Acid Transport Systems, Neutral / Q000737:chemistry*; D000818:Animals; D003545:Cysteine / Q000737:chemistry; D003554:Cystinosis / Q000378:metabolism; D004195:Disease Models, Animal; D005721:Glutamate-Cysteine Ligase / Q000737:chemistry / Q000502:physiology*; D018698:Glutamic Acid / Q000737:chemistry; D006801:Humans; D051379:Mice; D008954:Models, Biological; D011752:Pyroglutamate Hydrolase / Q000737:chemistry / Q000502:physiology*; D011761:Pyrrolidonecarboxylic Acid / Q000737:chemistry
D026921:Amino Acid Transport Systems, Neutral; C111445:CTNS protein, human; C534463:cystinosin protein, mouse; D018698:Glutamic Acid; D000255:Adenosine Triphosphate; D011752:Pyroglutamate Hydrolase; D005721:Glutamate-Cysteine Ligase; D003545:Cysteine; D011761:Pyrrolidonecarboxylic Acid