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Yeast mutants unable to degrade certain nitrogen compounds produce characteristic small red colonies on an agar medium containing the red dye phloxine B, galactose, the test nitrogen compound, and a small amount of ammonium chloride.
Two major nitrogenous waste products, urea and ammonium (NH(4)(+)), are produced in humans when proteins are oxidized, and in this manuscript their excretions are examined from two perspectives. First, the specific physiology of each nitrogenous waste is reviewed and the current dogmas summarized. Second, their excretions are considered in the context of integrative physiology, i.e. the need to ensure that the urine composition is appropriate to minimize the risk of kidney stone formation. After the latter analysis, weak links in our understanding of the overall physiology become apparent and a conundrum is defined. The conundrum for the excretion of urea focuses on the fact that urea is not an effective osmole in the medullary-collecting duct when vasopressin acts. As a result, it appears that urinary urea cannot prevent a large decline in the urine flow rate and thereby minimize the risk of forming kidney stones in electrolyte-poor urine. The conundrum for the excretion of NH(4)(+) is: high rates of NH(4)(+) excretion require a low urine pH, yet a pH approximately 6.0 must be maintained in order to reduce the risk of precipitating uric acid in the urine. Possible ways of resolving these conundrums require novel physiological interpretations.
Nitrogen (N) limitation of primary production is common in temperate salt marshes, even though conservative N recycling can fulfill a large proportion of plant N demand. In nutrient poor young marshes, N limitation may be more severe and new N sources, such as N fixation, more important for plant growth. We measured N fixation and the response of salt marsh primary producers (Spartina alterniflora and benthic microalgae) to N fertilization in one mature (>150 year) and two young (7 and 15 year) naturally developing marshes at the Virginia Coast Reserve LTER site. S. alterniflora aboveground biomass in the mature marsh (1,700+/-273 g m(-2)) was 1.8 and 2.8 times higher than in the 15 year and 7 year old marshes, respectively. Fertilization significantly increased S. alterniflora biomass in the two young marshes (160-175%) and areal aboveground tissue N in the youngest marsh (260%). Microalgal chlorophyll a (Chl a) in the mature marsh was nearly 2-fold lower than in the 7-year-old marsh, and there was no evidence that this was due to light limitation. However, Chl a in fertilized plots was 30% higher than control plots at the youngest site. Daily N fixation decreased with increasing marsh age in summer, when rates were highest at all sites. Autotrophic N fixation (difference between rates in the light and dark) was most important in the summer, but we saw no indication of a shift in dominance between autotrophic and heterotrophic N fixers during marsh development. Estimated annual N fixation was 2- to 3-fold higher in the young marsh (18.3+/-1.5 g N m(-2) year(-1)), than in the intermediate-aged (9.0+/-0.7) or mature marsh (6.1+/-0.5). In the young marshes, N fixation was sufficient to provide a substantial proportion of aboveground S. alterniflora N demand. Our results suggest that both benthic microalgae and S. alterniflora in young salt marshes are N limited, and that this limitation decreases as the marsh matures. The high rates of N fixation by autotrophic and heterotrophic bacteria in the sediment could provide an important source of N for primary producers during marsh development.
The objective of the present study was to elucidate whether remobilized N from lower leaves is involved in causing the drop in N(2) fixation during pod-filling in common bean (Phaseolus vulgaris L). Moreover, we addressed the question of whether remobilized N from lower leaves would reach the nodules. Nodulated common bean plants were grown in a growth chamber in quartz sand. During a 2-week period, at vegetative and at reproductive growth, 50% of the leaves (lower part) were either excised or individually darkened, thereby removing the same photosynthetic capacity yet allowing N to be remobilized from the darkened leaves. Moreover, at the vegetative growth period, three lower leaves per plants were (15)N labelled by applying (15)NH(4)NO(3) prior to imposing the darkening treatment. Leaf darkening at vegetative growth induced N remobilization as well as reduced N(2)-fixation rates and growth. Leaf excision at reproductive growth enhanced N(2) fixation. Changes in N(2)-fixation rates were in all cases the result of altered growth rates, while the % N in the whole plant and in various plant parts remained conserved. Directly after leaf labelling, but also at the end of the vegetative growth period, substantial amounts of (15)N from the leaves could be recovered in nodules in the control, and in higher amounts in the leaf-darkening treatment. It is proposed that nitrogen from leaves circulates within the plant via nodules, and that the strength or composition of this circular flow may be the signal for a putative N-feedback effect.
Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) seedlings were grown in a 2 x 2 factorial design in enclosed mesocosms at ambient temperature or 3.5 degrees C above ambient, and at ambient CO2 concentration ([CO2]) or 179 ppm above ambient. Two additional mesocosms were maintained as open controls. We measured the extent of mycorrhizal infection, foliar nitrogen (N) concentrations on both a weight basis (%N) and area basis (Narea), and foliar delta15N signatures (15N/14N ratios) from summer 1993 through summer 1997. Mycorrhizal fungi had colonized nearly all root tips across all treatments by spring 1994. Elevated [CO2] lowered foliar %N but did not affect N(area), whereas elevated temperature increased both foliar %N and Narea. Foliar delta15N was initially -1 per thousand and dropped by the final harvest to between -4 and -5 per thousand in the enclosed mesocosms, probably because of transfer of isotopically depleted N from mycorrhizal fungi. Based on the similarity in foliar delta15N among treatments, we conclude that mycorrhizal fungi had similar N allocation patterns across CO2 and temperature treatments. We combined isotopic and Narea data for 1993-94 to calculate fluxes of N for second- and third-year needles. Yearly N influxes were higher in second-year needles than in third-year needles (about 160 and 50% of initial leaf N, respectively), indicating greater sink strength in the younger needles. Influxes of N in second-year needles increased in response to elevated temperature, suggesting increased N supply from soil relative to plant N demands. In the elevated temperature treatments, N effluxes from third-year needles were higher in seedlings in elevated [CO2] than in ambient [CO2], probably because of increased N allocation below ground. We conclude that N allocation patterns shifted in response to the elevated temperature and [CO2] treatments in the seedlings but not in their fungal symbionts.
Nitrogen (N) saturation is an environmental concern for forests in the eastern U.S. Although several watersheds of the Fernow Experimental Forest (FEF), West Virginia exhibit symptoms of N saturation, many watersheds display a high degree of spatial variability in soil N processing. This study examined the effects of temperature on net N mineralization and nitrification in N-saturated soils from FEF, and how these effects varied between high N-processing vs. low N-processing soils collected from two watersheds, WS3 (fertilized with [NH4]2SO4) and WS4 (untreated control). Samples of forest floor material (O1 horizon) and mineral soil (to a 5-cm depth) were taken from three subplots within each of four plots that represented the extremes of highest and lowest rates of net N mineralization and nitrification (hereafter, high N and low N, respectively) of untreated WS4 and N-treated WS3: control/low N, control/high N, N-treated/low N, N-treated/high N. Forest floor material was analyzed for carbon (C), lignin, and N. Subsamples of mineral soil were extracted immediately with 1 N KCl and analyzed for NH4+ and NO3- to determine preincubation levels. Extracts were also analyzed for Mg, Ca, Al, and pH. To test the hypothesis that the lack of net nitrification observed in field incubations on the untreated/low N plot was the result of absence of nitrifier populations, we characterized the bacterial community involved in N cycling by amplification of amoA genes. Remaining soil was incubated for 28 d at three temperatures (10, 20, and 30 degrees C), followed by 1 N KCl extraction and analysis for NH4+ and NO3-. Net nitrification was essentially 100% of net N mineralization for all samples combined. Nitrification rates from lab incubations at all temperatures supported earlier observations based on field incubations. At 30 degrees C, rates from N-treated/high N were three times those of N-treated/low N. Highest rates were found for untreated/high N (two times greater than those of N-treated/high N), whereas untreated/low N exhibited no net nitrification. However, soils exhibiting no net nitrification tested positive for presence of nitrifying bacteria, causing us to reject our initial hypothesis. We hypothesize that nitrifier populations in such soil are being inhibited by a combination of low Ca to Al ratios in mineral soil and allelopathic interactions with mycorrhizae of ericaceous species in the herbaceous layer.
In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO2] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO2] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. In a field free-air CO2 enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO2] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution 15N methods were used to estimate N fixation. Although N addition did, in general, decrease N fixation, contrary to theoretical predictions, elevated [CO2] did not universally increase N fixation. Rather, the effect of elevated [CO2] on N fixation was positive, neutral or negative, depending on the species and N addition. Our results suggest that legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric [CO2].
We investigated nutrient limitations during hydrocarbon degradation in a sandy soil and found that fixed nitrogen was initially a limiting nutrient but that N limitation could sometimes be overcome by N2 fixation. Hydrocarbon biodegradation was examined in an unsaturated sandy soil incubated aerobically at 20 degrees C with propane or butane and various added nutrients. Propane and butane degradation proceeded similarly during the first 3 months of incubation. That is, bacteria in soil amended with N oxidized these hydrocarbons more rapidly than in controls without nutrient additions or in soil with added phosphate or trace minerals. Both propane- and butane-amended soil apparently became N limited after the initial available inorganic N was utilized, as indicated by a decrease in the rates of hydrocarbon degradation. After 3 months, propane and butane degradation proceeded differently. Bacteria in propane-degrading soil apparently remained N limited because propane degradation rates stayed low unless more N was added. In contrast, bacteria in butane-degrading soil appeared to overcome their N limitation because butane degradation rates later increased regardless of whether more N was added. Analyses of total N and acetylene reduction assays supported this apparent surplus of N in butane-amended soil. Total N was significantly (P < 0.01) higher in soil incubated with butane and no N amendments than in soil incubated with propane, even when the latter was amended with N. Acetylene reduction occurred only in butane-amended soil. These results indicate that N2 fixation occurred in butane-amended soil but not in propane-amended soil.
Previous studies have observed higher levels of soluble nutrients leaving vegetative buffers than entering them, suggesting that the buffers themselves are acting as a source rather than a sink by releasing previously stored nutrients. This study used 98 atom % (15)N-labeled KNO(3) at a rate of 5 kg ha(-1) to quantify buffer efficiency for sequestering new inputs of NO(-)(3)-N in an extensively grazed irrigated pasture system. Buffer treatments consisted of an 8-m buffer, a 16-m buffer, and a nonbuffered control. Regardless of the form of runoff N (NO(-)(3), NH(+)(4), or dissolved organic nitrogen [DON]), more (15)N was lost from the nonbuffered treatments than from the buffered treatments. The majority of the N attenuation was by vegetative uptake. Over the course of the study, the 8-m buffer decreased NO(-)(3)-(15)N load by 28% and the 16-m buffer decreased load by 42%. For NH(+)(4)-(15)N, the decrease was 34 and 48%, and for DON-(15)N, the decrease was 21 and 9%. Although the buffers were effective overall, the majority of the buffer impact occurred in the first four weeks after (15)N application, with the buffered plots attenuating nearly twice as much (15)N as the nonbuffered plots. For the remainder of the study, buffer effect was not as marked; there was a steady release of (15)N, particularly NO(-)(3)- and DON-(15)N, from the buffers into the runoff. This suggests that for buffers to be sustainable for N sequestration there is a need to manage buffer vegetation to maximize N demand and retention.
Studies were done to derive a technique to quantify intraruminal recycling of microbial nonammonia nitrogen (NAN). After dosing 15NH4+ into the rumen, ruminal NH3N and NAN pools were sampled over time for analyses of 15N enrichment. Compartmental analysis of both pools was used to quantify the percentage of microbial NAN that recycles in the rumen. Microbial NAN does not need to be fractionated from total NAN, assuming that only microbial NAN becomes labeled with 15N and that the ratio of microbial NAN:nonmicrobial NAN remains constant over time. Based on the data obtained from eight nonlactating cattle fed 85% corn silage diets at 1.5 to 1.0% BW, percentage of intraruminal recycling of microbial NAN was related to efficiency of bacterial protein synthesis (grams of CP/100 g of OM truly digested) in a curvilinear manner (y = 100 - .073x2). From this function, recycling was predicted to be approximately 75% for dairy cows consuming 50:50 forage:concentrate diets at 3.5% of BW. However, more data are needed to evaluate intraruminal recycling of microbial NAN using this technique under different dietary conditions.
A 2-yr study was conducted to evaluate 1) the effects of nitrogen level from swine lagoon effluent on forage composition and animal performance and 2) the effects of supplemental escape protein (EP) on performance by steers grazing pastures fertilized with swine lagoon effluent. Bermudagrass (Cynodon dactylon) pastures were fertilized via sprinkler irrigation with either 448 or 896 kg/ha of N from swine lagoon effluent (two pasture replications/treatment). Within each pasture, four supplement treatments were evaluated using electronic Calan gates (8 steers.pasture-1.yr-1). The treatments were 1) negative control (pasture only); 2) energy control, which supplied 79.9 g of EP.hd-1.d-1; 3) 159 g of EP.hd-1.d-1, and 4) 239 g of EP.hd-1.d-1. Gains were increased (P < .05) an average of .15 kg/d by supplementation, with no differences in gain among supplements. Forage samples representative of that grazed by steers (CONSUM) and representative of all available forage (AVAIL) were obtained at 14-d intervals. Total CP content of CONSUM and AVAIL samples were slightly higher (P < .20 and P < .15, respectively) from pastures fertilized with 896 compared with 448 kg/ha of N (20.8 vs 20.0% for CONSUM and 15.2 vs 14.2% for AVAIL). Concentrations of nonprotein N and soluble true protein (% of CP) in both AVAIL and CONSUM samples were higher (P < .06) from pastures fertilized with the higher N level. Total N uptake by the plant, based on CP content and animal grazing days/ha, was essentially the same for both N treatments.
Urea recycling in ruminants has been studied extensively in the past, but the mechanisms regulating the amount of urea recycled or excreted remain obscure. To elucidate the role of urea transporters (UT) in N recycling, nine Dorset-Finn ewe lambs (20.8 +/- 0.8 kg) were fed diets containing 15.5, 28.4, and 41.3 g of N/kg of DM for 25 d. Nitrogen balance and urea N kinetics were measured during the last 3 d of the period. Animals were then slaughtered and mucosa samples from the rumen, duodenum, ileum, and cecum, as well as kidney medulla and liver, were collected. Increasing N intake tended to increase N balance quadratically (1.5, 5.1, and 4.4 +/- 0.86 g of N/d, P < 0.09), and linearly increased urinary N excretion (2.4, 10, and 16.5 +/- 0.86 g N/d, P < 0.001) and plasma urea N concentration (4.3, 20.3, and 28.4 +/- 2.62 mg of urea N/dL, P < 0.001), but did not affect fecal N excretion (5.0 +/- 0.5 g of N/d; P < 0.94). Urea N production (2.4, 11.8, and 19.2 +/- 0.83 g of N/d; P < 0.001) and urinary urea N excretion (0.7, 7.0, and 13.4 +/- 0.73 g N/d; P < 0.001) increased linearly with N intake, as well as with the urea N recycled to the gastrointestinal tract (1.8, 4.8, and 5.8 +/- 0.40 g of N/d, P < 0.001). No changes due to N intake were observed for creatinine excretion (518 +/- 82.4 mg/d; P < 0.69) and clearance (46 +/- 10.7 mL/min; P < 0.56), but urea N clearance increased linearly with N intake (14.9, 24.4, and 34.9 +/- 5.9 mL/min; P < 0.04). Urea N reabsorption by the kidney tended to decrease (66.3, 38.5, 29.1 +/- 12.6%; P < 0.06) with increasing N content of the diet. Increasing the level of N intake increased linearly the weight of the liver as a proportion of BW (1.73, 1.88, and 2.22 +/- 0.15%, P < 0.03) but only tended to increase the weight of the kidneys (0.36, 0.37, and 0.50 +/- 0.05%, P < 0.08). Urea transporter B was present in all the tissues analyzed, but UT-A was detected only in kidney medulla, liver, and duodenum. Among animals on the three diets, no differences (P > 0.10) in UT abundance, quantified by densitometry, were found. Ruminal-wall urease activity decreased linearly (P < 0.02) with increasing level of N intake. Urease activity in duodenal, ileal, and cecal mucosa did not differ from zero (P > 0.10) in lambs on the high-protein diet. In the present experiment, urea transporter abundance in the kidney medulla and the gastrointestinal tract did not reflect the increase in urea-N reabsorption by the kidney and transferred into the gut.
Effects of increased ammonia and/or arginine absorption across the portal-drained viscera (PDV) on net splanchnic (PDV and liver) metabolism of nitrogenous compounds and urinary N excretion were investigated in six catheterized Hereford x Angus steers (501 +/- 1 kg BW) fed a 75% alfalfa:25% (as-fed basis) corn-soybean meal diet (0.523 MJ of ME/[kg BW(0.75).d]) every 2 h without (27.0 g of N/kg of dietary DM) and with 20 g of urea/kg of dietary DM (35.7 g of N/kg of dietary DM) in a split-plot design. Net splanchnic flux measurements were obtained immediately before beginning and ending a 72-h mesenteric vein infusion of L-arginine (15 mmol/h). For 3 d before and during arginine infusion, daily urine voided was measured and analyzed for N composition. Feeding urea increased PDV absorption (P < 0.01) and hepatic removal (P < 0.01) of ammonia N, accounting for 80% of increased hepatic urea N output (P < 0.01). Numerical increases in net hepatic removal of AA N could account for the remaining portion of increased hepatic urea N output. Arginine infusion increased hepatic arginine removal (P < 0.01) and hepatic urea N output (P < 0.03) and switched hepatic ornithine flux from net uptake to net output (P < 0.01), but numerical changes in net hepatic removal of ammonia and AA N could not account fully for the increase in hepatic urea N output. Increases in urine N excretion equaled quantities of N fed as urea or infused as arginine. Estimated salivary urea N excretion was not changed by either treatment. Urea cycle regulation occurs via a complex interaction of mechanisms and requires N sources other than ammonia, but the effect of increased ammonia absorption on hepatic catabolism of individual AA in the present study was not significant.
Two experiments with a randomized complete block design were conducted to determine the effects of phase feeding of CP on performance, blood urea nitrogen (BUN), manure N:P ratio, and carcass characteristics of steers fed in a feedlot. In Exp. 1, 45 crossbred steers (initial BW = 423 +/- 3.3 kg) were individually fed a diet formulated to contain 13.0% CP (DM basis) for 62 d. On d 63, the dietary CP was maintained at 13.0% or formulated to contain 11.5 or 10.0% CP until slaughter. Actual CP values were 12.8, 11.8, and 9.9%, respectively. Reducing the CP concentration of the diet did not affect ADG of steers from d 62 to 109 (P = 0.54) or over the 109-d feeding period (1.45, 1.50, and 1.49 kg/d for 13.0, 11.5, and 10.0% CP, respectively; P = 0.85). No differences (P > 0.12) among treatments were detected for BUN concentrations on d 0, 62, or 109. Gain:feed, DMI, and carcass characteristics did not differ among treatments (P > 0.10). In Exp. 2, 2 trials were conducted using 184 (initial BW = 406 +/- 2.6 kg) and 162 (initial BW = 342 +/- 1.9 kg) crossbred steers. Data from the 2 trials were pooled for statistical analysis, and trial effect was added to the statistical model. Steers were fed a diet formulated to contain 13.0% CP until reaching approximately 477 kg. When the average BW of the pen was 477 kg, diets were maintained at 13.0% CP or reduced to contain 11.5 or 10.0% CP. Actual CP values were 12.4, 11.5, and 9.3% CP for treatments 13.0, 11.5, and 10.0% CP, respectively. Reducing the CP content of the diet did not affect ADG after the diet changed (P = 0.16) or throughout the finishing period (P = 0.14). Immediately before slaughter, steers fed the 13.0% CP diet had greater (P < 0.001) BUN concentrations than steers fed the 11.5 and 10.0% CP diets. Carcasses from cattle fed the 11.5% CP diet had greater (P = 0.02) fat thickness than the 13.0 and 10.0% CP treatments, whereas carcasses from cattle fed 13.0% CP had greater (P = 0.004) marbling scores than steers fed the 11.5 or 10.0% CP diets. Other carcass characteristics, DMI, and G:F did not differ (P > 0.10) among treatments. The N:P ratio was increased with the 10.0% CP diet (P = 0.02) compared with the 11.5 or 13.5% CP treatments; however, manure composition did not differ (P > 0.10) among treatments. These results indicate that reduced CP concentration during the finishing period does not affect feedlot performance but can improve the N and P relationship in the manure.
Eight Angus heifer calves (234 kg) were assigned to either a high (HP; 126 g N/d) or low (LP; 66.5 g N/d) protein intake. Calves received 4.8 kg DM/d consisting of 30% cottonseed hulls and 70% corn-soybean meal in equal portions at 4-h intervals. Single doses of 14C- and 15N-urea and 15N-ammonium sulfate were injected into the blood urea-N (BUN) and ruminal NH3-N (RAN) pools, respectively, to measure rate of flux through, and transfer of N between, these and bacterial N. Nitrogen balance was greater (P less than .05) for HP than for LP (56.9 vs 25.1 g N/d), but abomasal N flow as a percentage of N intake was greater (P less than .05) for LP than for HP (124 vs 71.1%). Pool size and net synthesis rate for both RAN and BUN pools were greater (P less than .05) for HP than for LP. Calves fed HP degraded more (P less than .05) BUN in the gastrointestinal tract than calves fed LP (37.4 vs 14.0 g N/d). Quantities of RAN absorbed from the rumen also were greater (P less than .05) for HP than for LP (14.2 vs 2.8 g N/d). The proportion of total gastrointestinal BUN degradation occurring in the rumen averaged 53 and 26% for LP and HP. Data are interpreted to suggest that net incorporation of BUN into bacterial protein (urea recycling) is inversely related to level of protein intake.
The concept of the single pool of metabolic nitrogen in measuring the rate of whole-body protein turnover has been examined both with the practical evidence which is available in the literature and from a theoretical point of view. It is concluded that for the most part the concept is not valid. The main problem is one of introducing the isotope uniformly throughout the body. It is suggested that better results may be obtained if the 15N tracer is given simultaneously by both the oral and intravenous route.
High-latitude peatlands are important soil carbon sinks. In these ecosystems, the mineralization of carbon and nitrogen are constrained by low temperatures and low nutrient concentrations in plant litter and soil organic matter. Global warming is predicted to increase soil N availability for plants at high-latitude sites. We applied N fertilizer as an experimental analogue for this increase. In a three-year field experiment we studied N fertilization effects on leaf litter decomposition and N dynamics of the four dominant plant species (comprising >75% of total aboveground biomass) in a sub-arctic bog in northern Sweden. The species were Empetrum nigrum (evergreen shrub), Eriophorum vaginatum (graminoid), Betula nana (deciduous shrub) and Rubus chamaemorus (perennial forb). In the controls, litter mass loss rates increased in the order: Empetrum<Eriophorum<Betula<Rubus. Increased N availability had variable, species-specific effects: litter mass loss rates (expressed per unit litter mass) increased in Empetrum, did not change in Eriophorum and Betula and decreased in Rubus. In the leaf litter from the controls, we measured no or only slight net N mineralization even after three years. In the N-fertilized treatments we found strong net N immobilization, especially in Eriophorum and Betula. This suggests that, probably owing to substantial chemical and/or microbial immobilization, additional N supply does not increase the rate of N cycling for at least the first three years.
Conflicting studies have been reported regarding the influence of thyroid hormones on hepatic nitrogen metabolism and liver metabolic activity. We studied urea N synthesis rate (UNSR), functional hepatic N clearance (FHNC), galactose elimination capacity, and antipyrine clearance in six hypothyroid female patients before and after achievement of a stable euthyroid status. In both conditions, UNSR measured at intervals in response to constant alanine infusion was linearly related to the average alpha-amino N concentrations. In the hypothyroid state, peak UNSR was decreased by 31% in comparison with values measured in euthyroidism, which were in the normal range. FHNC (ie, the slope of the linear relation between UNSR and blood alpha-amino N concentration) is a measure of the kinetics of the process of hepatic amino N to urea N conversion; it was 19.8 +/- 4.0 L.h-1 in hypothyroid patients and increased to normal values after L-thyroxine replacement (30.4 +/- 3.3 L.h-1, P < .01; normal values > 25 L.h-1). Hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hypothyroid patients and did not change significantly after therapy. Our data show a specific defect in hepatic handling of amino acids in hypothyroid patients, leading to reduced alpha-amino N to urea N conversion, in the absence of any detectable impairment in different hepatic metabolic activities.