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Urinary Molecular Forms of Human N-Terminal of Pro Opiomelanocortin: Possible Deglycosylation and Degradation by the Kidney^*

LASZLO GASPAR, JOHN S. D. CHAN, NABIL G. SEIDAH and MICHEL CHRÉTIEN

Laboratories of Biochemical and Molecular Neuroendocrinology, Clinical Research Institute of Montreal Montreal, Quebec, Canada H2W1R7

Address correspondence and requests for reprints to: Dr. Michel Chretien, Laboratory of Molecular Neuroendocrinology, Clinical Research Institute of Montreal, 110 Pine Avenue West, Montreal, Quebec H2W 1R7, Canada.

The NH₂-terminal fragment (hNT) of proopiomelanocortin¹ is found predominantly as one molecular form of apparent mol wt of 12K in the circulation. Since the kidney may play an important role in the elimination and degradation of proopiomelanocortin-related peptides, we analyzed the urinary forms of immunoreactive hNT (IR-hNT) by molecular sieving and carbohydrate affinity (Concanavalin A-agarose) chromatography. RIA specific for the amino terminal portion and for the ₃-MSH (carboxy-terminal portion of hNT) were used in these studies. Molecular sieve chromatography revealed several forms of IR-hNT in the urine from normal subjects, patients with Nelson's syndrome, and patients with ectopic ACTH-secreting tumors. A considerable decrease in IR-hNT and IR-₃-MSH was found in the urine of a patient with ACTH deficiency and normal subjects during glucocorticoid suppression. In urine from normal subjects and a patient with lung cancer not causing Cushing's syndrome, the majority of aminoterminal IR-hNT (66–83%) had apparent mol wts of 3–4K, 6–7K, and 8–10K, and did not cross-react with the ₃-MSH antiserum. Ten to nineteen percent of the total IR-hNT was eluted in the position of authentic hNT and reacted with the ₃-MSH RIA. In patients with Nelson's syndrome and those with ectopic ACTH syndrome, almost no intact hNT (less than 1% of the total) was present in urine; most of the IR-hNT appeared in the elution volumes with an apparent mol wt of 8–10K. In addition, smaller forms (6–7K and 3–4K) of hNT were also detected in the urine of these patients. The major form of urinary IR-₃-MSH exhibited an apparent mol wt of 7–8K and did not correspond to any of the peaks of IR-hNT. Carbohydrate affinity chromatography (Concanavalin A-agarose) of smaller forms of IR-hNT revealed weak affinity to the lectin, which suggests loss of the carbohydrate moiety during renal excretion. We conclude that hNT in urine is present in extensively cleaved forms and that deglycosylation may be an important step in hNT degradation. These results support a role for the kidney in the catabolism of hNT.

^* This work was supported by the National Center Institute of Canada, La Fondation de l'Hotel-Dieu de Montreal, and La Foundation de Seve.

Postdoctoral fellow from Endocrine Unit, First Department of Medicine, Szeged University Medical School, Szeged, Hungary; Recipient of a fellowship of the Cancer Research Society Inc.

Received January 23, 1984.