Amino acid composition of human milk is not unique

Arginine nutrition in development, health and disease

As a precursor of nitric oxide, polyamines and other molecules with enormous biologic importance, L-arginine plays versatile key roles in nutrition and metabolism. Arginine is an essential amino acid in the fetus and neonate, and is conditionally an essential nutrient for adults, particularly in certain disease conditions. L-Arginine administration is beneficial in improving reproductive, cardiovascular, pulmonary, renal, gastrointestinal, liver and immune functions, and in facilitating wound healing. The effect of L-arginine in treating many common health problems is unique among amino acids, and offers great promise for improved health and well-being in the future.

Proline ameliorates arginine deficiency during enteral but not parenteral feeding in neonatal piglets

The indispensability of arginine has not been conclusively established in newborns. Because parenteral feeding bypasses the gut (where de novo synthesis of arginine occurs from proline), a dietary supply of arginine that is sufficient to maintain urea cycle function may be of greater importance during intravenous compared with enteral feeding. Two-day-old piglets (n = 12) were fed nutritionally complete diets for 5 days via either a central vein catheter (IV pigs, n = 6) or a gastric catheter (IG pigs, n = 6). Subsequently, each piglet received three incomplete test diets [arginine free (-ARG/+PRO), proline free (-PRO/+ARG), or arginine and proline free (-ARG/-PRO)] in a randomized crossover design. Plasma ammonia was assayed every 30 min for 8 h or until hyperammonemia was observed. Ammonia increased rapidly in IV pigs receiving -ARG/+PRO and -ARG/-PRO (84 +/- 36 and 74 +/- 37 micromol. l(-1). h(-1), respectively), requiring early diet cessation. A rapid increase was also exhibited by IG pigs receiving the -ARG/-PRO, but not the -ARG/+PRO diet (31 +/- 15 vs. 11 +/- 7 micromol. l(-1). h(-1), respectively, P < 0.05). Plasma arginine and proline were indicative of deficiency (IG and IV groups) when deplete diets were infused. Arginine is indispensable in parenteral and enteral nutrition, independent of dietary proline.

Lactate inhibits citrulline and arginine synthesis from proline in pig enterocytes

Hypocitrullinemia and hypoargininemia but hyperprolinemia are associated with elevated plasma concentration of lactate in infants. Because the small intestine may be a major organ for initiating proline catabolism via proline oxidase in the body and is the major source of circulating citrulline and arginine in neonates, we hypothesized that lactate is an inhibitor of intestinal synthesis of citrulline and arginine from proline. To test this hypothesis, jejunum was obtained from 14-day-old suckling pigs for preparation of enterocyte mitochondria and metabolic studies. Mitochondria were used for measuring proline oxidase activity in the presence of 0-10 mM L-lactate. For metabolic studies, enterocytes were incubated at 37 degrees C for 30 min in Krebs bicarbonate buffer (pH 7.4) containing 5 mM D-glucose, 2 mM L-glutamine, 2 mM L-[U-14C]proline, and 0, 1, 5, or 10 mM L-lactate. Kinetics analysis revealed noncompetitive inhibition of intestinal proline oxidase by lactate (decreased maximal velocity and unaltered Michaelis constant). Lactate had no effect on either activities of other enzymes for arginine synthesis from proline or proline uptake by enterocytes but decreased the synthesis of ornithine, citrulline, and arginine from proline in a concentration-dependent manner. These results demonstrate that lactate decreased intestinal synthesis of citrulline and arginine from proline via an inhibition of proline oxidase and provide a biochemical basis for explaining hyperprolinemia, hypocitrullinemia, and hypoargininemia in infants with hyperlactacidemia.

The nutritional value of plant-based diets in relation to human amino acid and protein requirements

The adequacy of plant-based diets in developed and developing countries as sources of protein and amino acids for human subjects of all ages is examined. Protein quantity is shown not to be an issue. Digestibility is identified as a problem for some cereals (millet (Panicum miliaceum) and sorghum (Sorghum sp.)) and generally is poorly understood. Direct measurements of biological value in children are reviewed and scoring is considered. Various existing requirement values for amino acids and especially lysine are reviewed, and it is concluded that stable-isotope studies do not yet provide adequate alternative values of N balance data, which for lysine are robust after recalculation and adjustment. A new maintenance requirement pattern is developed, with higher values than those of Food and Agriculture Organization/World Health Organization/United Nations University (1985) but lower values than the Massachusetts Institute of Technology pattern (Young et al. 1989). Calculations of age-related amino acid requirements are based on most recent estimates of human growth and maintenance protein requirements, a tissue amino acid pattern and the new maintenance amino acid pattern. These values appear valid when used to score plant proteins, since they indicate values similar to or less than the biological value measured directly in young children. When used to score plant-based diets in India, no marked deficiencies are identified. All regions score > 1 for adults, whilst for children scores range from > 1, (Tamil Nadhu) from 6 months of age to 0.78 (West Bengal), rising to 0.9 in the 2-5 year old, consistent with reports that high-lysine maize supports similar weight and height growth to that of casein. Inadequate amino acid supply is not an issue with most cereal-based diets.

Arginine metabolism: nitric oxide and beyond

Arginine is one of the most versatile amino acids in animal cells, serving as a precursor for the synthesis not only of proteins but also of nitric oxide, urea, polyamines, proline, glutamate, creatine and agmatine. Of the enzymes that catalyse rate-controlling steps in arginine synthesis and catabolism, argininosuccinate synthase, the two arginase isoenzymes, the three nitric oxide synthase isoenzymes and arginine decarboxylase have been recognized in recent years as key factors in regulating newly identified aspects of arginine metabolism. In particular, changes in the activities of argininosuccinate synthase, the arginases, the inducible isoenzyme of nitric oxide synthase and also cationic amino acid transporters play major roles in determining the metabolic fates of arginine in health and disease, and recent studies have identified complex patterns of interaction among these enzymes. There is growing interest in the potential roles of the arginase isoenzymes as regulators of the synthesis of nitric oxide, polyamines, proline and glutamate. Physiological roles and relationships between the pathways of arginine synthesis and catabolism in vivo are complex and difficult to analyse, owing to compartmentalized expression of various enzymes at both organ (e.g. liver, small intestine and kidney) and subcellular (cytosol and mitochondria) levels, as well as to changes in expression during development and in response to diet, hormones and cytokines. The ongoing development of new cell lines and animal models using cDNA clones and genes for key arginine metabolic enzymes will provide new approaches more clearly elucidating the physiological roles of these enzymes.

Catabolism dominates the first-pass intestinal metabolism of dietary essential amino acids in milk protein-fed piglets

To investigate the extent of first-pass intestinal metabolism of dietary amino acids, seven female pigs (28 d old, 8.0 kg) were implanted with arterial, venous, portal and gastric catheters and with an ultrasonic portal blood flow probe. The pigs were fed a milk-based diet once hourly and infused intragastrically with [U-13C]algal protein. On average, 56% of the essential amino acid (EAA) intake appeared in the portal blood. However, the net portal balance of methionine (48% of intake) and threonine (38% of intake) tended (P = 0.08) to be lower than the mean of all EAA. The net portal balance (expressed as a percentage of intake) of alanine (205%), tyrosine (167%) and arginine (137%) exceeded their intake. Net portal outflow of ammonia accounted for 18% of total amino acid nitrogen intake. As a percentage of the enteral tracer input, there was substantial first-pass metabolism of lysine (35%), leucine (32%), phenylalanine (35%) and threonine (61%). However, only 18, 21, 18 and 12% of the total first-pass metabolism of lysine, leucine, phenylalanine and threonine, respectively, were recovered in mucosal protein. We conclude that roughly one third of dietary intake of EAA is consumed in first-pass metabolism by the intestine and that amino acid catabolism by the mucosal cells is quantitatively greater than amino acid incorporation into mucosal protein.

Free amino acids in milks of human subjects, other primates and non-primates

Preterm and term transitional milks of human subjects and mature milks of human subjects, non-human primates and non-primates were analysed for free amino acids (AA) using precolumn phenylisothiocyanate derivatization and liquid chromatography. Differences in free AA between three types of human milk were small. Milks of pinnipeds (seals and sea lions) contained the highest levels of total free AA (8634-20,862 mumol/l), while the milks of cows and sheep had the lowest levels of total free AA (1061-1357 mumol/l). The milks of human subjects, chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), elephants (Elephas maximus), horses and pigs had intermediate levels of total free AA (3069-7381 mumol/l). Glutamic acid was the most abundant free AA in milks of human subjects (1339-2157 mumol/l), non-human primates (423-2528 mumol/l), elephants (1332 mumol/l), horses (1119 mumol/l), and cows (349 mumol/l). Taurine was the most abundant free AA in milks of pinnipeds (5776-13,643 mumol/l), pigs (1238 mumol/l), goats (1150 mumol/l) and sheep (341 mumol/l). Taurine was the second most abundant free AA in milks of human subjects and non-human primates, while histidine was the second most abundant free AA in milks of pinnipeds. Milks of each species had a distinctive free AA pattern which may reflect the relative importance of the free AA during early postnatal development.

Phenylalanine utilization by the gut and liver measured with intravenous and intragastric tracers in pigs

To investigate intestinal and hepatic metabolism of phenylalanine, four conscious pigs (7.5 kg), bearing arterial, venous, and hepatic portal catheters, were fasted for 12 h and infused with [phenyl-2H5]phenylalanine via a peripheral vein and [carboxyl-13C]phenylalanine via the stomach. During the first 6 h of the infusion, the pigs remained fasted and received only the intravenous tracer. During the second 6 h, they received an intragastric infusion of milk replacer and both tracers. In the fasted state, the portal-drained viscera extracted 10% (P < 0.025) of the arterial [2H5]phenylalanine flow of the pigs. In the fed state, the splanchnic tissues metabolized 45% of the enteral tracer and intestinal metabolism accounted for 76% of the total splanchnic extraction. The tracer-to-tracee ratio of both tracers in apolipoprotein B-100 (apo B-100) phenylalanine was twofold (P < 0.001) higher than that of hepatic free phenylalanine. The ratios of the two tracers in portal (13C/2H; 1.66) and apo B-100 (1.76) phenylalanine were similar but higher (P < 0.05) than that of arterial phenylalanine (1.29). We conclude that intestinal metabolism dominates the splanchnic extraction of enteral phenylalanine and that in the fed state, the hepatic protein synthetic precursor pool derives from portal phenylalanine.

Endogenous synthesis of arginine plays an important role in maintaining arginine homeostasis in postweaning growing pigs

This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweaning pigs. Pigs were fed a sorghum-based diet containing 0. 98% arginine and were used for studies at 75 d of age (28.4 kg body weight). Mitochondria were prepared from the jejunum and other major tissues for measuring the activities of Delta1-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (the enzyme catalyzing the interconversion of P5C into ornithine). For metabolic studies, jejunal enterocytes were incubated at 37 degrees C for 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-glutamine, 2 mmol/L L-[U-14C]proline, and 0-200 micromol/L gabaculine (an inhibitor of OAT). The activities of P5C synthase, proline oxidase and OAT were greatest in enterocytes among all of the tissues studied. Incubation of enterocytes with gabaculine resulted in decreases (P < 0.05) in the synthesis of ornithine and citrulline from glutamine and proline. When gabaculine was orally administered to pigs (0.83 mg/kg body weight) to inhibit intestinal synthesis of citrulline from glutamine and proline, plasma concentrations of citrulline (-26%) and arginine (-22%) decreased (P < 0.05), whereas those of alanine (+21%), ornithine (+17%), proline (+107%), taurine (+56%) and branched-chain amino acids (+21-40%) increased (P < 0.05). On the basis of dietary arginine intake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided >/=50.2% of total daily arginine requirement. Taken together, our results suggest an important role for endogenous synthesis of arginine in regulating arginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs.

Glutamine supplementation maintains intramuscular glutamine concentrations and normalizes lymphocyte function in infected early weaned pigs

Numerous studies in humans and rats have shown that glutamine supplementation during stressful conditions has favorable outcomes. However, the requirements for glutamine during weaning are unknown. Thus, the effects of glutamine supplementation in healthy and infected weaned pigs were investigated. At 21 d of age, pigs were weaned to an elemental diet supplemented with glutamine (+Gln) or an isonitrogenous diet containing nonessential amino acids (-Gln). At 26 d of age, pigs were intraperitoneally injected with Escherichia coli (+Ecoli) or buffered saline (-Ecoli) and killed at 28 d of age. Infection decreased (P < 0.05) plasma and intramuscular glutamine concentrations, but infected pigs that received +Gln diets had higher intramuscular glutamine levels than those that received -Gln diets. Infected pigs had elevated (P < 0.05) total leukocyte counts, and blood lymphocyte responses ([3H]-thymidine incorporation) to a mixture of phorbol myristate acetate and ionomycin were reduced. White blood cell counts were greater (P < 0.05) in +Gln than -Gln pigs. The peak responses to concanavalin A (Con A) by lymphocytes of +Ecoli+Gln pigs were greater (P < 0.05) than those of +Ecoli-Gln pigs and not different than those of noninfected pigs. Hence, glutamine supplementation maintained muscular glutamine concentrations and normalized lymphocyte function in infected pigs.

Enteral glutamate is almost completely metabolized in first pass by the gastrointestinal tract of infant pigs

We studied the absorption of enteral glutamate and phenylalanine using isotopic tracer and arteriovenous difference techniques. Six piglets, implanted with portal, carotid, and gastric catheters and an ultrasonic portal flow probe received a 6-h intragastric infusion of [U-13C] glutamate and [2H] phenylalanine, with a high-protein diet offered one time each hour. Amino acid concentrations and the isotopic enrichments of all mass isotopomers of glutamate, glutamine, and phenylalanine were measured in portal and arterial blood over the last hour. There was significant (P<0.025) net absorption of the indispensable amino acids as well as arginine, proline, serine, and alanine. There was no portal uptake of glutamate, aspartate, and glycine, and arterial glutamine was removed by the portal drained viscera (P<0.05). At isotopic steady state, 72% of the [2H] phenylalanine but only 5% of the [U-13C] glutamate tracer appeared in the portal blood. We conclude that, in fed infant pigs, the gut metabolizes virtually all of the enteral glutamate during absorption. Therefore, glutamate and glutamine in the body as a whole must derive almost entirely from synthesis de novo.

Amino acid composition of pinniped milk

The total amino acid concentration and the amino acid pattern, i.e. the relative proportion of each amino acid (protein-bound plus free) to the total amino acids, in the milks of the Northern elephant seal, Antarctic fur seal, California sea lion, and Australian sea lion were determined. Total amino acid concentration was 10% (w/v) or greater and did not vary significantly among species. The most abundant amino acids in the milks of all species were glutamate, proline and leucine. Essential amino acids were 40%, branched-chain amino acids were 20%, and sulfur amino acids were 4% of the total milk amino acids in all species. There were differences among the pinnipeds in some of the individual amino acids; the milk of the Northern elephant seal was the most distinct among the pinnipeds with higher histidine, serine and cystine contents and a lower methionine content than that of other pinnipeds. There was little effect of stage of lactation on total amino acid concentration or amino acid pattern in pinniped milk. Comparison of milk from the four pinniped species with that of 14 other mammalian species suggests commonality in milk amino acid pattern despite the wide variation in total amino acid concentration among the species.

Amino acid composition of the milk of some mammalian species changes with stage of lactation

To determine whether the amino acid composition of milk changes during lactation, we compared the amino acid pattern (concentration of each individual amino acid relative to the total amino acid concentration) of colostrum with that of mature milk in six mammalian species. In the human, horse, pig and cow, the pattern of amino acids changed between colostrum and mature milk: glutamate, proline, methionine, isoleucine and lysine increased; cystine, glycine, serine, threonine and alanine decreased. In these four species, the total amino acid concentration also decreased 75% between colostrum and mature milk. In the baboon (Papio cynocephalus anubis and Papio cynocephalus anubis/Papio cynocephalus cynocephalus) and rhesus monkey (Macaca mulatta), however, there was little change in the pattern of amino acids between colostrum and mature milk, and total amino acid concentration decreased only about 25% between colostrum and mature milk. Mature milk rather than colostrum was the most similar among the three primates in both amino acid pattern and total amino acid concentration. We conclude, in those species in which total amino acid concentrations decline substantially between colostrum and mature milk, amino acid patterns also change. The presence of a change in amino acid pattern and total amino acid concentration during lactation appears to be unrelated to phylogenetic order.