Urea kinetic modelling in two groups of patients on continuous ambulatory peritoneal dialysis

Dialysis adequacy (Kt/V) was investigated in two groups of patients on continuous ambulatory peritoneal dialysis (CAPD). Group I consisted of patients with serum creatinine concentration above 1200 umol/l and Group II comprised patients with serum creatinine concentration of 600 umol/l and less. The mean Kt/V was significantly higher in Group II (Kt/V, 2.0) than in Group I (Kt/V, 1.59; p < 0.01) patients. The mean duration of CAPD was significantly longer in Group I (3.12 years) than in Group II (1.32 years); (p < 0.01) patients, and the mean total creatinine clearance of Group II patients was significantly higher than for Group I (p < 0.001) patients. There was good correlation between Kt/V and total creatinine clearance (r = 0.73; p < 0.001); and between Kt/V and normalized protein catabolic rate (NPCR, r = 0.6; p < 0.001). There was weak correlation between Kt/V and duration on dialysis, but this was statistically significant. There was no significant difference between Kt/V and duration on dialysis, but this was statistically significant. There was no significant difference between mean NPCR and mean mid-arm muscle circumference (MAMC) in the two groups and no significant association between Kt/V and dietary inventory. Group II patients had a significantly better residual renal clearance (p < 0.0001). Pruritus was a troublesome feature in Group I patients but in both groups patients were distressed by loss of libido, insomnia and tiredness. This study revealed that Group II patients with lower creatinine concentrations had better dialysis adequacy but were on CAPD for a shorter duration than Group I and had significantly better residual renal clearance and total clearance. Muscle mass does not appear to have contributed significantly to the differences in creatinine concentration between the groups. Additional studies on peritoneal membrane function vis-a-vis solute transfer are in progress.(AU)

Urea kinetics varies in Jamaican women and men in relation to adiposity, lean body mass and protein intake

OBJECTIVE: We have measured urea kinetics in normal adult men and women of different body composition to determine whether adiposity is associated with differences in the rate of urea production or endogenous urea hydrolysis. DESIGN: Urea kinetics were determined from the excretion of [15N15N] urea in urine over a period of 48 h following a single oral dose of [15N15N] urea, in nine lean and nine obese women and in seven light and seven heavy males while they were consuming their habitual diets. Urinary 5-L-oxoproline was measured as an index of glycine metabolic status. SETTING: The studies were carried out in the research ward of the Tropical Metabolism Research Unit, University of the West Indies. RESULTS: Successful studies were completed in eight obese and five lean women and in six heavy and five light men. When compared with lean women, in obese women the rate of urea production and hydrolysis was significantly greater and this difference could not be accounted for by the greater fat-free mass alone, and was in part associated directly with the increase in fat mass. The rate of urea production and hydrolysis was greater in heavy men than in light men, a difference which was attributed to an increase in dietary protein. In obese women and heavy men there was a significantly higher rate of excretion of 5-Loxoproline in urine when compared with lean women and lean men respectively. CONCLUSION: This paper highlights the difficulty in identifying an appropriate reference with which to express results in people of different body composition. In obese women urea production and the hydrolysis of urea are increased, in part related to the increase fat-free mass, but also related to the increased fat mass itself. In obese women and men on high protein diets the greater rate of hydrolysis urea may be a reflection of an increased demand for the sythesis of non-essential amino acids, especially glycine.(AU)

Urea production and salvage during pregnancy in normal Jamaica women

The pattern of aggregated nitrogen demand during pregnancy and the fetal and maternal components are unclear. Excess demand enhances efficiency of nitrogen utilization. Urea salvage contributes to enhanced efficiency. Dietary protein intake, urea production, and salvage of urea nitrogen were measured in eight nonpregnant control subjects, and trimesterly in nine pregnant women. Production was measured after prime-intermittent intravenous doses of [15N15N]-urea by dilution of label in urinary urea. Dietary protein intake was greater in trimester 1 than in nonpregnant women (167 ñ 36 vs 224 ñ 60 mg N.kg-1.d-1), and increased further in trimester 2 (266 ñ 59 mg N.kg-1.d-1). Urea production was not higher during pregnancy. Despite higher protein intake urea salvage was higer in pregnancy (40 ñ 24 nonpregnant vs 77 ñ 23, 61 ñ 31, and 51 ñ 12 mg N.kg-1.d-1). Therefore, the demand-supply gap for nitrogen was greatest early in pregnancy when fetoplacental growth is slowest, and implies heightened maternal demand (AU)

The effect of the level of dietary protein, carbohydrate and fat on urea kinetics in young children during rapid catch-up weight gain

The kinetics of urea metabolism were measured in children recovering from severe malnutrition. For a period of up to 10 d they receive one of four diets which provided 711 kj (170 kcal)/kg per d. Two groups received a diet with a high protein:energy (P:E) ratio of 10.6 percent (HP), enriched with either fat (HP/F) or maize starch and sucrose HP/C). Two groups received a diet with a low P:E ratio of 8.8 percent (LP), enriched with either fat (LP/F) or maize starch and sucrose (LP/C). The rate of weight gain on the HP diets was significsntly greater than on the LP diets. There was no difference in urea production between any of the four diets: HP/F 1.23 (se 0.12), HP/C 1.37 (se 0.14), LP/F 1.64 (se 0.22) LP/C 1.15 (se 0.15) mmol nitrogen/kg per h. On the HP diets urea excretion was 0.77 (se0.07) mmol N/kg per h, 61 percent of production. There was significantly less urea excreted in the urine on diet LP/C than on LP/F (0.36 (se0.05) and 0.64 (se 0.04)mmol N/kg per h respectively). A significantly greater percentage of the urea production was hydrolysed on the LP diets (61 percent) compared with the HP diets (39 percent), with the consequence that 50 percent of urea-N produced was available for synthetic activity on the LP diets compared with 30 percent on the HP diets. The increase in the urea hydrolysed on the LP diets was equivalent in magnitude to the decreased intake of N, so that overall intake plus hydrolysis did not differ between the LP and HP diets. Crude N balance was similiar on diets HP/F, HP/C and LP/C, but was significantly reduced on diet LP/F. These results show that there is an accommodation in urea kinetics during rapid catch-up weight gain, which becomes evident when the P:E ratio of 8.8 percent, protein is limiting for catch-up growth. When the intake has a P:E ratio of 8.8 percent the pattern of urea kinetics can be modified by the relative proportion of fat and carbohydrate in the diet. The measurement of urea kinetics provides a useful approach to the definition of the adequacy of the protein in the diet. (AU)

Urea kinetics in sickle cell disease

When dietary nitrogen intake is adequate to satisfy the body's metabolic demand for nitrogen, adaptive mechanisms are involved whereby nitrogen losses are minimised. The most significant reduction in nitrogen loss is effected by reduced urea production and excretion and there is a concomitant increase in the proportion of urea hydrolysed and recycled in the body. This work was designed to explore the mechanisms by which nitrogen equilibrium is maintained in the whole body when the metabolic demands for nitrogen exceed the supply. Urea kinetics was measured in normal healthy controls (HbAA) and patients with homozygous sickle cell disease (HbSS). HbSS is characterised by an increased metabolic demand for nitrogen for erythropoesis, which is significantly higher than normal. Urea production (P), excretion (Eu), hydrolysis (T) and recycling (Pr) of hydrolysed urea to urea synthesis, were determined using isotopic urea(urea-30) orally or intravenously, and the two methods compared. The reliability of the methods was examined and demonstrated to be satisfactory. All aspects of urea metabolism were increased in HbSS compared with HbAA, except Eu which tended to be lower in HbSS. The proportion of urea hydrolysed in the colon was raised in HbSS, regardless of the route by which the isotope was given and the kinetics was measured. Individuals with sickle cell trait (HbAS), were also studied and exhibited rates of urea hydrolysis that were either similar to the HbAA values or raised to values corresponding to the HbSS. Urea nitrogen incorporated into haemoglobin in both the HbAA and HbSs. This finding confirmed the utilisation of endogenous urea nitrogen for protein synthesis in the body. The proportion of hydrolysed urea nitrogen is small (10 percent) compared to a larger pool of enteric nitrogen to which it contributes, suggesting that there may be a more significant contribution of enteric nitrogen metabolism to the nitrogen economy, than of hydrolysed urea alone. The possibility that glycine may be limited in HbSS has been indicated and warrants more detailed investigation. The metabolism of urea has been shown to be affected by depleting the body glycine pool and alternatively by glycine supplements. Results from this work support the idea that urea kinetics is modulated by dietary nitrogen intake (AU)

Whole-body protein turnover in man determined in three hours with oral or intravenous 15N-glycine and enrichment in urinary ammonia

Studies were carried out in eight normal adults to simplify the continuous infusion-end product method for measuring whole-body protein turnover using 15 N-glycine. When a priming dose of label suitable for the urea pool was followed by intermittent oral doses of label, plateau enrichment was maintained in urinary urea and ammonia from 9 to 18 h, giving values for nitrogen flux. (18h) of 0.69ñ0.05 g N/kg/d with urea and 0.46ñ0.01 g N/kg/d with ammonia. With a priming dose appropriate for the ammonia pool, plateau was reached in urinary ammonia in less than 120 min an maintained for up to 6h. Nitrogen flux (3h) with oral 15N-glycine was 0.96ñ0.12 g N/kg/d, and with intravenous label was 0.61ñ0.13 g N/kg/d. There was a significant linear relationship between flux measured with oral and intravenous isotope. It is suggested that different components of protein turnover are measured with the different approaches, and that the short method in particular measures rapidly turning over proteins associated with the gastrointestinal tract.(AU)

The non-invasive measurement of urea kinetics in normal man by a constant infusion of 15N15N-urea

A two-pool model is described for the non-invasive measurement of urea kinetics in man. The isotope, 15N-urea, was given until an isotopic steady state was reached in urine and the time taken achieve this is defined. During an isotopic steady state, a comparison was made of the effect of giving the isotope orally, intravenously and intragastrically; no differences were found between the different routes. Measurements of enrichment were made on excretion products in urine. In six normal adults with a protein intake of 200 mg N/kg/d, the urea production rate was 139 ñ 15 mg N/kg/d, 70 percent of which was excreted in urine. Of the 34 mg N/kg/d produced by hydrolysis of urea in the gastrointestinal tract, 41 percent was resynthesized to urea, and about 48 percent was available for other synthetic processes.(AU)

Sodium, potassium, and rate constants for sodium efflux in leucocytes from hypertensive Jamaicans

Leucocyte sodium and potassium content and concentrations were measured along with ouabain-sensitive and ouabain-insensitive rate constants for sodium efflux in 14 controls and 20 black patients with essential hypertension. Leucocyte sodium content was significantly increased in the patients (mean 101.1 ñ 7.8 mmol/kg dry solids v 74.5 ñ 7.6 mmol/kg dry solids; p<0.05), whereas the rate constants for sodium efflux were not significantly reduced. There was no difference between the two groups in cell potassium values. The increase in leucocyte sodium content in the presence of normal rate constants for sodium efflux suggests an increase in membrane permeability to sodium, which might be important in the pathogenesis of essential hypertension. (AU)

Fasting glucose production in the smaller of twins with epinephrine-deficient hypoglycemia

The possibility that insufficient glucose production or availability of gluconeogenic substrates could account for fasting hypoglycemia was investigated in three children with epinephrine deficiency. Each had been born the smaller of discordant identical twins, and the unaffected twins served as controls. Fasting plasma glucose production was measured by constant infusion of U-[13]C-glucose under steady-state conditions and was compared with availability of potential glucose sources estimated from respiratory calorimetry and excretory nitrogen. The average rate of glucose production was 2.6 mg/kg/min in the affected twins after they became symptomatic and 2.9 mg/kg/min in the control twins after comparable fasting. Plasma alanine was lower in the affected twins during this interval (average: 0.11 mM versus 0.16 mM), but not earlier prior to decreased plasma glucose; alanine correlated with plasma glucose in a similar way in both groups (r = 0.77). Plasma urea production was 0.30 versus 0.15 mg urea N/kg/min. The calculated availability of potential gluconeogenic amino acids was 1.2 versus 0.6 mg/kg/min. Availability of glycerol, estimated from respiratory calorimetry was 0.4 mg/kg/min in both groups. In two of the twin pairs, net oxidation of carbohydrate (glycogen) was, by design, relatively small under these conditions (0.1 and 0.4 mg/kg/min in the affected and control twins, respectively). Gluconeogenesis therefore accounted for the majority of glucose production. The unaccounted remaining major gluconeogenic source is assumed to be recycled substrates from unoxidized pyruvate. Infusion of excess alanine in these two pairs increased plasma glucose and glucose production similarly in both the affected and control twins. This change was associated with an abnormally large increase in plasma alanine. In the third twin pair, net oxidation of carbohydrate was greater in the affected twin (1.8 versus 1.3 mg/kg/min) and possible glucose sources exceeded total glucose production during hypoglycemia. Earlier during fasting, net oxidation of carbohydrate in this twin was 5.8 mg/kg/min versus 3.1 mg/kg/min in the control. Plasma glucose production measured simultaneously was 4.3 versus 3.8 mg/kg/min, being less than the rate of carbohydrtae oxidation in the affected twin. It is concluded that the abnormal fasting metabolism observed in these children with decreased epinephrine was not primarily a consequence of deficient glucose production or lack of potential gluconeogenic substrates. Initial persistent oxidation of glycogen and subsequent increased utilization of protein during hypoglycemia indicate failure to conserve these limited net sources of pyruvate(AU)

The relationship between dietary intake, weight change, nitrogen balance, and protein turnover in man

We have examined the relationships between protein turnover, protein synthesis, and protein breakdown and dietary intake, weight change, and nitrogen balance in children who were recovering and had recovered from severe protein-energy-malnutrition. Protein metabolism was measured by giving [15N]glycine and measuring the enrichment of urinary area. The levels of dietary protein did not affect protein metabolism. There were highly significant correlations between both protein flux and protein synthesis and the ad libitum dietary intake, nitrogen balance, and weight change. Over the range of dietary intake, 60 to 270 cal/kg per day, the protein synthesis rate increased 5-fold. Large changes in dietary intake resulted in small changes in protein breakdown, with breakdown being least on an inadequate intake. Changes in the rate of protein breakdown did not contribute to changes in nitrogen balance or body weight. (AU)

Modification of diamine oxidase activity in vitro by metabolites of asparagine and differences in asparagine decarboxylation in normal and virus-transformed baby hamster kidney cells

The oxidation of putrescine in vitro by pig kidney diamine oxidase (EC 1.4.3.6) was increased in the presence of 2-oxosuccinamic acid and malonamic acid. It was inhibited by 3-aminopropionamide, oxaloacetate and pyruvate. 2-Oxosuccinamate was derived from asparagine in virus-transformed baby hamster kidney (BHK) cells growing in tissue culture. Asparagine was decarboxylated more efficiently by transformed than by normal BHK cells. In BHK cells transformed by polyoma virus (Py BHK), 2-oxosuccinamate is the most likely immediate precursor of the 14 CO2 arising from [U-14C] asparagine, and there was some evidence for its formation in an asparagine-dependent clone of BHK cells before and after their transformation by hamster sarcoma virus (respectivey Asn- and HSV Asn-). The relationship between 2-oxosuccinamate and pyruvate and the possible roles of these two substances in controlling cellular diamine oxidase activity are discussed (AU)

The reactions of haemoglobins in intact red cells and in solution

Haemoglobin is a fascinating substance with a number of orders of structure. At one time it was thought that the red cell was an inert bag full of haemoglobin. But we know that the haemoglobin in red cells has an active metabolism in oxygen transport in the body. Crystallographers, biochemists, physical chemists, physicists, biophysicists, physiologists, geneticians and scientists from many disciplines of medicine have been actively studying the structure and function of this protein and its role in oxygen transport for more than a hundred years. Many other chemical properties of the protein have also been investigated and an enormous literature is available on the subject. Some thirty to forty papers from all over the world are currently being published every month on various aspects of haemoglobin behaviour. Since the discovery of sickle cell anaemia and of the abnormal haemoglobin, Hb S, some 210 abnormal haemoglobins have been discovered and the properties of many of them have been investigated in detail. In the last few years, a remarkable series of studies have been performed on the metabolism of the red cell as an organ of gas transport; for example, the presence of organic phosphates in the red cell and their effects on oxygen affinity have been discovered. The abnormal haemoglobin, Hb S, is particularly interesting. Deoxygenated haemoglobin S from the blood of patients with homozygous sickle cell anaemia aggregates into fibres and crystallises in the erythrocytes and the cells becomes rigid and elongated. A number of differences between the behaviour of sickle haemoglobin and that of normal haemoglobin in the red cell have been demonstrated. Much data of apparent simplicity has been accumulated for haemoglobins in solution, but the situation is much more complex in the cell, where the haemoglobin concentration is 100- or 500- fold greater; intertetramer interactions must become more important within the erythrocyte. A comparative study, presented in this thesis, has been made of some of the equilibrium and kinetic behaviour of normal and sickle haemoglobin both in aqueous solution and in the intact erythrocyte. SECTION I contains a review of the pertinent literature. The experimental methods and the materials employed, and the methods of calculation used, are described in SECTION II. SECTION III contains the results, and the discussion of these results, of a number of different but related experiments which are mostly concerned with the equilibrium and kinetic behaviour of normal and sickle haemoglobin in solution and in the red cell and of the simple derivatives of these haemoglobins (AU)

Rhein, a selective inhibitor of the DPNH-flavin step in mitochondrial electron transport

Previous findings in the literature that rhein inhibits DPNH-linked mitochondrial oxidations by acting in the DPNH dehydrogenase region of the respiratory chain have been confirmed and extended. In the micromolar range rhein inhibits DPNH oxidase and DPNH-ferricyanide activities and the energy-linked reduction of DPNH by succinate in membrane preparations from heart, as wellas the DPNH dehydrogenase and transhydrogenase activities of the soluble, purified enzyme. The inhibition of the activities of the soluble enzyme are purely competitive with respect to substrate. These facts localize the primary inhibition site of rhein between substrate and FMN. In heart ETP a second noncompetitive inhibition is also present but is detectable only at very low (<10æM) rhein concentrations. Rhein also inhibits DPNH dehydrogenase in Candida utilis mitochondria and the purified enzyme from liver. On conversion of the heart enzyme to the low molecule weight DPNH-cytochrome reductase the typical effect of rhein disappears and is replaced by a slight stimulation or inhibition, depending on the electron acceptor used, showing that the substrate binding site is modified in this form of the enzyme. In beef liver mitochondria DPNH oxidation may appear insensitive to rhein, probably because of the strong binding of rhein to other proteins. To a lesser extent unspecific binding of rhein and resultant interference with the inhibition of DPNH dehydrogenase is also shown by BSA and by proteins in heart ETP. Rhein also inhibits transhydrogenations in mitochondria and at higher concentrations lactate and malate dehydrogenases but has no effect on sccinate, alcohol (liver nad yeast), and glucose-6-p dehydrogenases or on Neuospora DPN-ase, glucose-6-phosphatase, and amine oxidase. (SUMMARY)