Renal tubular absorption of the free amino acids L-proline, hydroxy-L-proline, and glycine in man is accomplished by a transport system exhibiting preference for this group of amino acids (1, 2). The characteristics of saturability (for L-proline transport) and selective inhibition between the constituent substrates have been demonstrated for this system in man (1). Studies in vivo in the rat (3, 4) also reveal a transport system in that animal with preference for the same three amino acids. Studies in vitro with rat kidney cortex slices (4) and other tissues (5, 6) confirm this finding and also indicate that, within the system showing group preference, there may be discrete catalytic transport sites for the individual substrates (4, 6). The apparent order of substrate affinities for the transport system or systems is highest for L-proline and lowest for glycine (1, 4).A maximal rate for renal tubular absorption of L-proline (Tm Pro) has been shown in man (1). In the present work the behavior of free hydroxy-L-proline as substrate and its effect as an inhibitor within the system were studied. A Tm for hydroxyproline (Tm Hypro) was demonstrated; the imino acid was also shown to be a poor inhibitor of L-proline transport but more effective against glycine.* Submitted for publication November 8, 1965; accepted May 20, 1966. Supported by grant AM-05117-04 from the Division
Methods
SubjectsFour healthy adult Caucasian males between 30 and 39 years of age were studied. Two subjects (2 and 3) were given two infusions; the remaining two were given one infusion. Techniques Infusions were performed in the morning with the subjects in the resting state after an overnight fast. Sufficient water was given by mouth to ensure a diuresis exceeding 5 ml per minute. Urine was collected by spontaneous voiding while subjects were standing. The steady state infusion technique used in these experiments has been described previously (1). Hydroxy-L-proline was given in priming doses of 0.4 to 1.2 mmoles per kg (50 to 150 mg per kg) ; the sustaining dose was 3.5 /Amoles per kg per minute.Infusion of free hydroxy-L-proline is followed about 12 hours later by muscular ache, mild pyrexia, and nausea. These sensations disappear within 24 hours. This response is dose dependent, and the two subjects who were given two infusions felt greater discomfort with the larger infusion. Hydroxyprolinuria or persistence of hydroxyprolinemia were not observed at the time of symptoms. The origin of this reaction remains obscure. Materials Hydroxy-L-proline (chromatographically pure) was obtained commercially.1 Purity was confirmed by chromatographic analysis in n-butanol, acetic acid, water (12: 3: 5) followed by staining with Ninhydrin (0.2% in acetone) and sequential staining with isatin (0.2% in acetone) followed by Ehrlich's reagent (7).
Preparation of samplesAll urine samples were preserved with thymol and stored immediately after collection at -20°C. Plasma was separated from heparinized whole venous blood by centrifugation within an hour after...