Plasma Ornithine Levels Research Articles

Controlled study of enteral arginine supplementation in burned children: impact on immunologic and metabolic status

Objective We compared the effects of an arginine-supplemented diet with those of an isocaloric isonitrogenous diet on immune and metabolic response of children with burns. Methods This was a double-blind, randomized, placebo-controlled trial in a burn treatment center of a pediatric hospital in Santiago, Chile. All children (1–5 y of age) admitted within 48 h of a moderate to deep burn injury covering 10% to 40% of total body surface area were evaluated. Twenty-eight children met the criteria and were randomly assigned to receive an arginine-supplemented diet (AG; n = 14) or an isocaloric isonitrogenous diet (CG; control, n = 14) for 14 d. Samples were collected at admission (baseline) and on days 7 and 14 for lymphoproliferative response to mitogens, plasma interleukins (interleukin-1, interleukin-6, tumor necrosis factor-α), plasma arginine and ornithine levels, serum C-reactive protein, prealbumin, albumin, glucose, and total urinary nitrogen. Results The AG enhanced lymphoproliferative responses (analysis of variance, P < 0.05), which were 72% of normal at baseline in both groups; by day 7 responses increased to 144% in the AG group and decreased to 56% in the CG group; both groups returned to normal by day 14. Baseline interleukin-6 was significantly increased in all children. There were no differences in plasma concentrations of interleukin-1, interleukin-6, tumor necrosis factor-α, C-reactive protein, prealbumin, albumin, or glucose between the AG and CG groups. On day 7 plasma ornithine levels increased significantly in the AG versus CG group ( P < 0.05); arginine levels showed no change. Conclusions An exclusively AG improves mitogen-stimulated lymphocyte proliferation in burned children. The benefits of arginine for the immune system do not appear to be related to a metabolic response. The biological significance of this finding remains to be determined.

Dietary compliance in ornithine aminotransferase deficiency

Ornithine delta-aminotransferase (OAT) deficiency (McKusick 258870) is associated with hyperornithinaemia, thought to be the cause of the progressive retinal degeneration that occurs in this disorder. For the large majority of cases unresponsive to the co-factor pyridoxine, treatment is based on reducing ornithine plasma levels below 400 micromol/L with an arginine-restricted diet. This has been shown to slow the progression of retinal disease. (Santinelli et al 2004). In Table 1 we present our experience in the dietary management of 12 patients (7 female) from 8 families. Compliance was defined as good, intermediate or poor according to plasma ornithine levels. Only one patient could be categorized as a good complier, 5 were intermediate, and 6 were poor. The age at start of treatment was the most important factor as regards ability to comply with diet. Our study emphasizes the difficulty with dietary treatment and need for early diagnosis. For the older patients, alternative treatments such as the use of oral lysine to increase renal losses of ornithine need to be investigated further (Peltola et al 2000).

Use of an arginine-restricted diet to slow progression of visual loss in patients with gyrate atrophy.

To quantify the effect of long-term reduction of plasma ornithine levels through adherence to an arginine-restricted diet on visual function in patients of all ages with gyrate atrophy of the retina and choroid. A long-term observational study was conducted on 27 patients with gyrate atrophy, 17 of whom elected to comply with the arginine-restricted diet and 10 who were unable to comply. The mean rates of change in the electroretinogram combined response, electroretinogram flicker response, and kinetic and static perimetry were determined. After mean follow-up of 13.9 years for the patients on the diet and 14.1 years for those not on the diet, the mean rates of change for the diet group compared with those of the no-diet group were statistically significantly slower for all outcome measures (age-adjusted P<.05) except for static perimetry (P =.06). Adhering to an arginine-restricted diet so as to lower the plasma ornithine level below an average of 5.29 to 6.61 mg/dL (400-500 micromol/L) will slow the loss of function as measured by sequential electroretinography and visual field examinations.

Atrofia girata de coróide e retina: relato de caso

OBJETIVO: Relatar um caso de atrofia girata de coróide e retina com confirmação por meio da bioquímica do plasma. MÉTODO: Aferiu-se a melhor acuidade visual corrigida de ambos olhos (AO) em tabela de Snellen. Foram realizados biomicroscopia do segmento anterior, refração, mapeamento de retina, angiografia fluoresceínica, campo visual e dosagem da ornitina sérica (aminoacidograma). RESULTADOS: Paciente de 22 anos, sexo feminino, cor branca, apresentando alta miopia e acuidade visual (AV) 20/100 em AO. À biomicroscopia do segmento anterior apresentava catarata subcapsular posterior em AO. À oftalmoscopia foram verificadas lesões atróficas da coróide e da retina bem delimitadas em meia periferia de AO. O aminoacidograma constatou elevação correspondente ao complexo da ornitina. CONCLUSÃO: Relata-se um caso típico de atrofia girata, distrofia retiniana rara associada a hiperornitinemia.

Correction of ornithine accumulation prevents retinal degeneration in a mouse model of gyrate atrophy of the choroid and retina.

Deficiency of ornithine-delta-aminotransferase (OAT) in humans results in gyrate atrophy of the choroid and retina (GA), an autosomal recessive disorder characterized by ornithine accumulation and a progressive chorioretinal degeneration of unknown pathogenesis. To determine whether chronic, systemic reduction of ornithine can prevent this form of retinal degeneration, we used an arginine-restricted diet to maintain long term reduction of ornithine in a mouse model of OAT-deficiency (Oat(-/-)) produced by gene targeting. We evaluated the mice over a 12-month period by measurement of plasma amino acids, electroretinograms, and retinal histologic and ultrastructural studies. We found that an arginine-restricted diet substantially reduces plasma ornithine levels and completely prevents retinal degeneration in Oat(-/-). This result indicates that ornithine accumulation is a necessary factor in the pathophysiology of the retinal degeneration in GA and that restoration of OAT activity in retina is not required for effective treatment of GA.

Early plasma amino acid pool alterations in patients with military gunshot/missile wounds.

Plasma amino acid profiles in patients during the early period (first 18 hours) following military gunshot/missile wounds were investigated. Patients (n = 29) were casualties from the war in the former Yugoslavia with injury severity scores ranging from 4 to 18. They were divided into three groups: soft tissue (muscle) damage, wounds with fractures, and vital structures injured. Controls were normal blood donors (n = 17). Free amino acids were analyzed in venous plasma. Increased concentrations of phenylalanine and glutamine associated with increased molar phenylalanine/tyrosine ratio in plasma indicated increased net protein catabolism in the peripheral tissues, regardless of the type of injured tissues. Decreased plasma arginine, ornithine and citrulline levels, accompanied with increased molar glutamine/valine ratio, suggested disturbance in urea cycle activity, although urea level was not altered. We concluded that early changes in plasma amino acid pool characteristics after wounds were of systemic origin, not related to the type of injured tissues.

Growth retardation in children receiving long-term total parenteral nutrition: effects of ornithine alpha-ketoglutarate.

The authors evaluated the effect of supplementing parenteral nutrition (PN) with ornithine‐α‐ketoglutarate (OAK) on linear growth and biochemical parameters in a small group of prepubertal children requiring long‐term PN because of short bowel syndrome. Six study patients (5 male, 1 female) were selected from a group of 62 children receiving long‐term PN followed at a pediatric hospital in Paris, France. All of the patients had demonstrated significant growth retardation at entry (1 to 4 standard deviations below expected 50th percentile for height), and were prepubertal as evidenced by Stage I Tanner scores and prepubertal blood levels of testosterone, estradiol, and insulin‐like growth factor‐I (IGF‐1). The patients were determined to be normally nourished, as they received adequate doses of energy and nitrogen in PN prior to the study and demonstrated appropriate weight‐for‐height ratios, skinfold measurements and normal serum protein concentrations. The subjects had received constant dietary energy and nitrogen intakes for at least 8 months prior to study (primarily via PN).The usual PN prescription was continued during the entire 10 month experimental period. PN provided mean (± SD) energy and nitrogen doses of 242 ± 33 kJ/kg/d (~ 50 kcal/kg/d) and 324 ± 87 mg N/kg/d (~ 2.0 g protein/kg/ day) respectively, during each of two consecutive 5 month study periods. During the first 5 months of study, 15 g OAK was added to the daily PN solution bag immediately before the weekly home delivery of PN. During the second 5 months of study (control period), no OAK was added to the PN solution. Linear growth (height) velocities, body weight and plasma concentrations of amino acids, transferrin, retinol‐binding protein, prealbumin, albumin, total IGF‐1, testosterone and estradiol were measured monthly.During the 5‐month period of OAK supplementation of PN, linear growth velocity significantly increased to a median of 156% (range 126 to 186%) of baseline in five of the six subjects. During the subsequent 5 month control period (when OAK was not administered), three of these subjects demonstrated decreased height velocity (60%, 64% and 94% of baseline). The remaining two OAK responders evidently entered puberty during the post‐OAK control period, as their height velocities accelerated to 219% and 238% of baseline, their Tanner scores increased to stage 11, and their plasma IGF‐I and testosterone levels markedly rose. Overall, during OAK treatment, height velocity increased from a median of 3.80 cm/y to 6.45 cm/y (p &lt;.05) and decreased to a median of 3.65 cm/y in the subsequent OAK‐free period (p &lt;.05). Plasma IGF‐I concentrations rose significantly from a median of 200 U/L to 945 U/ L during OAK (p &lt;.05); IGF‐I levels fell after OAK in 3 of the 5 responders, but continued to rise in the two patients entering puberty.The change in IGF‐I level were positively correlated with the change in height velocity. No change in circulating proteins was observed. Although testosterone levels tended to rise in the 5 males studied during the OAK+ and OAK‐periods, levels remained prepubertal and no clear effect of OAK was discerned. No increase in plasma ornithine or other amino acid levels occurred during OAK, with the exception of plasma glutamine + glutamate, which rose by ~ 15% (from a mean of 621 to 715 mmol/L). The one subject who exhibited no increase in height response or IGF‐I levels during OAK had hepatic cirrhosis, and died of liver failure a few months after completion of study.

Substituting ornithine for arginine in total parenteral nutrition eliminates enhanced tumor growth.

Total parenteral nutrition (TPN) may enhance the growth of some tumors: this enhanced growth is associated with an increase in the erythrocyte polyamine levels. The effect of arginine in TPN on tumor growth was compared with ornithine using rats with a transplantable Ward colon tumor. The relationship of circulating arginine, ornithine, glutamine, and polyamines with tumor growth was investigated. For rats fed chow ad libitum, increasing tumor weights were associated with a linear decrease in the plasma arginine levels which was consistently lower than that of age-matched non-tumor bearing (NTB) rats; ornithine and lysine levels were not affected. Subsequent experiments suggest that plasma glutamine levels were also lower in tumor bearing rats. Pair-fed NTB rats had reduced arginine but not glutamine levels in plasma. TPN regimens with arginine or with ornithine substituted for arginine at two levels (equimolar [Orn-Em] or isonitrogenous [Orn-IN]) were given to colon tumor bearing rats for 8 days. The final tumor weight of rats which received the arginine-containing regimen (19.8 +/- 5.7 g, n = 4) (P less than 0.05) was significantly greater than the tumor weight of rats fed chow ad libitum (12.1 +/- 3.3 g, n = 6). The final tumor weights of Orn-EM (11.2 +/- 2.6 g, n = 4) or Orn-IN (11.6 +/- 0.8 g, n = 6) were similar to the chow-fed controls. The plasma arginine levels were elevated, compared with the control, when arginine was present in the regimen. The plasma arginine levels of rats which received Orn-EM or Orn-IN were lower than the controls. The plasma ornithine levels were not affected by arginine in the regimen but were elevated with increasing levels of ornithine in TPN. Plasma glutamine levels were decreased when arginine was present in the regimen but were elevated when ornithine was substituted for arginine. Erythrocyte putrescine was increased when either arginine or ornithine was included in the TPN regimens. These results demonstrate that while arginine in a parenteral regimen stimulates tumor growth, substituting ornithine for arginine in TPN does not enhance the growth of a transplantable colon tumor.

The metabolism and urinary excretion of orotic acid in hyperargininaemic sheep.

The synthesis, blood plasma turnover and urinary excretion of the orotic acid in normo- and hyperargininaemic sheep was investigated. The whole-body orotate formation was evaluated indirectly by the measurement of urinary orotate excretion after blockage of pyrimidine pathway with 6-azauridine (4 hour i.v. infusion of 0.2 mg.kg-1.min-1). Simultaneous infusion of L-arginine (2.5 mumols.kg-1.min-1) significantly elevated the blood plasma arginine, ornithine and urea level, however, it did not significantly influence urinary orotate excretion. In normoargininaemia blood plasma turnover of exogenous orotic acid amounted to 4.9 min and 67% of this compound was eliminated through the kidneys. The renal clearance of orotic acid amounted to 21.7 ml.min-1.kg-0.75. Hyperargininaemia elevated blood plasma turnover to 8.2 min, and diminished the renal clearance of this metabolite to 13.7 ml.min-1.kg-0.75. These results indicate that hyperargininaemia and hyperornithinaemia do not change the whole body synthesis of orotic acid in sheep but they can affect renal excretion of this metabolite, particularly at the rate of tubular secretion close to saturation.

A far advanced case of gyrate atrophy in a 12‐year‐old girl

Gyrate atrophy (GA; McKusick 25887) is a progressive chorioretinal dystrophy with an autosomal recessive mode of inheritance. In this disorder there is a deficiency of the mitochondrial matrix enzyme ornithine-f-aminotransferase (OAT). Ocular symptoms usually present late in the first decade of life. Ophthalmological findings include myopia, constricted visual fields, elevated dark adaptation thresholds (night blindness), very small or absent electroretinographic responses, and characteristic patchy chorioretinal atrophy distributed circumferentially around the peripheral fundus. Gradually the round patches of atrophy coalesce to form widespread chorioretinal atrophy extending to the posterior pole. Cataracts develop requiring surgical intervention, and eventually blindness occurs due to progressive loss of functional retina at the age of 40 to 50 (Valle and Simell, 1989). Plasma ornithine levels range from 400 to 1400/~mol/L, and 0.5 to 10mmol of ornithine is excreted daily. Glutamate, glutamine, lysine, creatine and creatinine concentrations in plasma are modestly reduced. The pathophysiological mechanism responsible for the chorioretinal dystrophy in GA is not known. Hyperornithinaemia or associated secondary metabolic abnormalities may play an important role, but until the pathophysiology is understood, there is no assurance that correction of biochemical abnormalities in plasma will actually be beneficial (Berson et al., 1981). Four general therapeutic approaches have been used: pharmacological doses of pyridoxine, reducing the intake of arginine and administration of creatine or proline. In this paper we described a 12-year-old patient with night blindness and myopia as the first symptoms of gyrate atrophy with ornithinaemia.

Episodic Hyperammonemia in Adult Siblings With Hyperornithinemia, Hyperammonemia, and Homocitrullinuria Syndrome

A 39-year-old man and his 42-year-old sister, both vegetarians, had episodic confusion for many years, but their mental function was normal between those episodes. They were recently diagnosed with hyperornithinemia, hyperammonemia, and homocitrullinuria syndrome. Hyperammonemia was documented during an episode of confusion in the male sibling but not in his sister. Both had elevated plasma ornithine, glutamine, and alanine levels and persistently low plasma lysine levels. Homocitrulline was present in their urine, and orotic aciduria and orotidinuria developed in the male sibling following ingestion of allopurinol. Studies on their cultured skin fibroblasts showed deficient metabolism of ornithine, indicating a defect in ornithine transport across the mitochondrial membrane. During therapy with citrulline and phenylbutyrate sodium, plasma ornithine levels increased in both patients, while plasma levels of glutamine and alanine decreased to normal. Since therapy started, their clinical conditions have also improved, and no recurrent neurologic dysfunction has occurred during a follow-up period of 20 months.

Plasma amino acid kinetics during acute states of glucagon deficiency and excess in healthy adults

The effects of glucagon deficiency and excess on plasma leucine, lysine, and alanine were examined in six healthy young adult men, with primed continuous infusions of L-[1-13C]- or L-[5,5,5-2H3]leucine, L-[alpha-15N]-lysine, and L-[3-13C]alanine for 150 min before and during 210 min of either a glucagon-deficient euglycemic state (experiment 1), a basal glucagon state (experiment 2), or a glucagon-excess state (experiment 3). Steady-state plasma hormone levels were achieved by infusion of somatostatin (250 micrograms/h) and insulin (0.07 mU.kg-1.min-1), without (experiment 1) or with an infusion of glucagon at 0.7 ng.kg-1.min-1 (experiment 2) or 2.5 ng.kg-1.min-1 (experiment 3). Plasma branched-chain amino acid (AA) concentrations did not change with altered glucagon status, whereas significant differences were observed for plasma lysine, alanine, glycine, serine, threonine, proline, tyrosine, citrulline, and ornithine levels (0.05 greater than P greater than 0.001). Plasma leucine, lysine, and alanine fluxes and the rate of de novo alanine synthesis showed no significant changes with either glucagon deficiency or excess. These findings lead to the conclusion that glucagon-induced alterations in plasma AA profiles are not due to changes in the rate of appearance of AA from peripheral tissues but rather a consequence of changes in the fate of AA within the splanchnic region.

Arginine-responsive asymptomatic hyperammonemia in the premature infant

We found that more than 50% of premature infants have elevated plasma ammonium levels during the first 2 months of life. Ammonium levels were twice normal and were unaccompanied by clinical symptoms of vomiting or lethargy. Ten of these infants were given supplements of arginine (1 to 2 mmol/kg/day PO) for 1 to 2 weeks preceded and followed by control periods. In each infant, plasma ammonium levels fell significantly within 2 days of start of arginine supplementation, and increased once arginine was discontinued. We studied 59 additional premature infants, of whom 26 had normal ammonium levels and 33 were hyperammonemic. Plasma arginine and ornithine levels were significantly lower in the hyperammonemic group, but there was no difference in urinary excretion of arginine or ornithine between groups. Half of the hyperammonemic infants received arginine supplementation between 2 and 8 weeks of age. Plasma ammonium levels in the arginine group was 33 + 1 mumol/L., compared to 45 + 2 mumol/L in the untreated group. Follow-up at 18 months of age showed similar IQ scores in all groups, suggesting that significant neurologic deficits do not result from this transient metabolic defect. The mechanism of the hyperammonemia is unclear.

Hyperammonemia in Lysinuric Protein Intolerance

Two brothers with hyperdibasicaminoaciduria and postprandial hyperammonemia showed characteristics of lysinuric protein intolerance. Intravenous alanine load produced hyperammonemia that was aborted by oral supplementation with arginine in one brother but not in the other, although both patients had almost the same intestinal malabsorption of arginine. This occurrence suggests that even a small amount of arginine, when absorbed into the blood, can normalize the affected ammonia metabolism of lysinuric protein intolerance. Two patients with cystinuria developed marked hyperammonemia when they received an intravenous alanine load after a 19-hour fast. As both patients displayed a reduced plasma concentration of arginine and ornithine at this time, the hyperammonemia was assumed to arise from the low plasma amino acid level. It seems likely that a decrease in plasma levels of urea cycle substrate causes a failure of the tissue urea cycle metabolism. Thus the impaired ammonia metabolism in lysinuric protein intolerance would be attributed to the low plasma arginine and ornithine levels.

Hyperargininemia: The rat as a model for the human disease and the comparative response to enzyme replacement therapy with free arginase and arginase-loaded erythrocytes in vivo

1. 1. Rat erythrocytes lack arginase as do the erythrocytes of human homozygote patients with hyperargininemia due to arginase deficiency. 2. 2. The rat has physiological liver arginase activity and plasma arginine and ornithine levels between the homozygotes and the hétérozygotes with hyperargininemia. 3. 3. In rats, one injection of free arginase induces a transient exogenous arginase effect which is abolished after 24 hr. One injection of isoionic arginase-loaded erythrocytes provokes an exogenous arginase effect in physiological “hyperargininemic” rats and pathological “hyperargininemic”-made rats for at least 8 and 5 days respectively. 4. 4. The very transient response in vivo to exogenous free arginase can be considerably prolonged by entrapment of the arginase in isoionic prepared erythrocytes.

Ascorbic acid, vitamin A, folic acid, and amino acids in blood of patients with hemophilia

Blood levels of ascorbic acid, vitamin A, folic acid, and amino acids were studied in patients at the South Texas Comprehensive Hemophilia Center, San Antonio, TX. The mean plasma ascorbic acid level in hemophiliacs was significantly lower than controls (p less than 0.0001). This was observed despite a dietary ascorbic acid intake in excess of 66% of the Recommended Dietary Allowances (RDA). However, those subjects receiving specific factor replacement therapy at home and consuming at least 66% RDA of ascorbic acid maintained a mean plasma ascorbic acid level not significantly less than controls. Hemophilic subjects not on home therapy, on the other hand, had a mean plasma ascorbic acid level significantly below that of controls while receiving optimal dietary ascorbic acid. With prompt adequate medical care of bleeding episodes and with optimal nutrition, the demand for ascorbic acid needed for tissue repair in hemophilic patients may be lessened. Hemophiliacs had mean serum vitamin A, mean serum folate, and mean red cell folate levels that were not significantly different from controls. Significantly higher mean plasma arginine and lower, but not significantly lower, mean plasma ornithine levels were found in hemophilic subjects, suggesting altered arginase activity.

Ascorbic Acid, Vitamin A, Folic Acid, and Amino Acids in Blood of Patients With Hemophilia

Abstract Blood levels of ascorbic acid, vitamin A, folic acid, and amino acids were studied in patients at the South Texas Comprehensive Hemophilia Center, San Antonio, TX. The mean plasma ascorbic acid level in hemophiliacs was significantly lower than controls (p less than 0.0001). This was observed despite a dietary ascorbic acid intake in excess of 66% of the Recommended Dietary Allowances (RDA). However, those subjects receiving specific factor replacement therapy at home and consuming at least 66% RDA of ascorbic acid maintained a mean plasma ascorbic acid level not significantly less than controls. Hemophilic subjects not on home therapy, on the other hand, had a mean plasma ascorbic acid level significantly below that of controls while receiving optimal dietary ascorbic acid. With prompt adequate medical care of bleeding episodes and with optimal nutrition, the demand for ascorbic acid needed for tissue repair in hemophilic patients may be lessened. Hemophiliacs had mean serum vitamin A, mean serum folate, and mean red cell folate levels that were not significantly different from controls. Significantly higher mean plasma arginine and lower, but not significantly lower, mean plasma ornithine levels were found in hemophilic subjects, suggesting altered arginase activity.

Long-term follow-up of some early-treated aminoacidopathies associated with ocular abnormalities

Experience in treating some aminoacidopathies associated with ocular abnormalities is reported. Sixteen out of 18 homocystinuric patients were treated with pyridoxine (300–1200 mg/day): 14 showed complete responsiveness, one was partially responsive, and one was a non-responder. In two early-diagnosed B6-responsive children treatment did not stop the progression of ocular abnormalities already present at the time of diagnosis. In one patient, completely asymptomatic at two years of age, a 5 1/2-year treatment with pyridoxine apparently prevented the appearance of either ectopia lentis or myopia. In a ten-year-old-girl affected by hyperornithinemia with gyrate atrophy of the choroid and retina, dietary treatment with a low-protein diet (0.8 g/kg per day) for more than six years reduced her plasma ornithine levels from 1189 to 774 μmol/1 and stopped the progression of the chorioretinal lesions. A short trial of pyridoxine given during the dietary treatment did not lead to a further reduction of her plasma o...

Reduction of hyperornithinemia with a low protein, low arginine diet and pyridoxine in patients with a deficiency of ornithine-ketoacid transaminase (OKT) activity and gyrate atrophy of the choroid and retina

Five patients with gyrate atrophy of the choroid and retina showed a 60% or greater decline in plasma ornithine levels during a five week trial of a low protein (10–15 g/day), low arginine (0.50–0.75 g/day) diet supplemented with essential amino acids and pyridoxine administration. These declines in plasma ornithine levels were seen in Patients 1–4 with the pyridoxine non-responsive variant and in Patient 5 with the pyridoxine responsive variant. No harmful systemic side effects were noted. Patients 1–4 continued on a modified low protein (20–35 g/day), low arginine (1.25–1.75 g/day) diet as tolerated and Patient 5 on pyridoxine alone. After one year no improvement was observed in visual acuities, visual fields, final dark adapted thresholds and full field electroretinograms for four patients. In contrast to the other four patients, Patient 3, with relatively poor control of plasma ornithine levels, showed signs of progression of the chorioretinal atrophy and further reduction of electroretinographic responses. Patients 1–4 continue on the dietary regimen and Patient 5 on pyridoxine alone to determine whether any lowering of plasma ornithine levels will modify the course of their ocular disease.

Lysinuric protein intolerance. Basolateral transport defect in renal tubuli.

In patients with an autosomal recessive diamino acid transport disorder, lysinuric protein intolerance (LPI), we measured plasma and urinary amino acids basally, and during intravenous infusion of citrulline at two rates. Compared with controls, the patients' plasma citrulline concentrations rose similarly, but urinary citrulline excretion increased excessively. Their plasma arginine and ornithine levels rose subnormally, but massive argininuria and moderate ornithinuria appeared. The excretion rates of the third diamino acid lysine and other amino acids remained practically unaltered, thus excluding mutual competition as the cause for the increases. The results suggest that (a) in the normal kidney reabsorption involves partial conversion of citrulline to arginine and ornithine (metabolic run-out), (b) in LPI, the diamino acid transport defect is located at the basolateral cell membrane of the renal tubules; this inhibits the efflux of arginine and ornithine, increasing their cellular concentration, which in turn inhibits the metabolic disposal of citrulline, and causes leakage of arginine, ornithine, and citrulline into the tubular lumen.