Update: dietary protein and microbial protein contribution

Factors influencing microbial protein synthesis in the rumen have been reviewed several times in recent years. Original publications in the past 3 yr have reported microbial and feed protein nitrogen contribution postruminally when feeding a variety of dietary proteins. Ammonia is a satisfactory source of nitrogen for growth of the majority of rumen species, but substitution of intact protein for urea (source of ammonia) usually stimulates microbial protein production. Protein sources such as soybean meal appear to possess properties (perhaps rate of degradability) that optimize microbial growth in vivo. Protein sources more undegradable than soybean meal, when fed as the major nitrogen source, sometimes reduce microbial growth. However, nondegradable proteins may compensate for less microbial protein by supplying intact dietary protein postruminally, so the amino acids potentially available may be equal to or greater than those available when readily degradable protein is fed. Soybean meal may reduce microbial growth in diets containing grass silage at protein exceeding 16.8%. Various measurements of microbial and intact dietary protein postruminally show that the contribution of each can be manipulated. Accuracy of quantitative predictions of postruminal contribution depends on several factors that require more research.     

Lipophilic soluble protein represents labile protein in rat liver.

Lipophilic proteins, having higher turnover rates than the average of the total soluble protein pool, were separated from soluble rat-liver proteins on alkyl agarose column. The quantitative immunoprecipitation, using the antibody against the lipophilic proteins, showed that the soluble proteins from the liver of fed rats contained about twice as much lipophilic proteins as that of 3-day-starved rats. This result indicates that the lipophilic proteins, at least in part, represent labile protein in rat liver.     

蛋白A-绿色荧光蛋白融合蛋白的构建与表达

绿色荧光蛋白 (GFP)是一种生物发光蛋白。GFP作为基因表达标记物或融合蛋白标记在动物、植物、微生物的研究中广泛应用。本实验将gfp基因与蛋白A基因重组 ,在大肠杆菌中融合表达为A GFP蛋白。根据gfp基因与质粒载体pRIT2T的序列 ,设计一对引物 ,以PCR技术扩增gfp基因序列 ,克隆到表达载体pRIT2T的EcoRⅠ与BamHⅠ酶切位点 ,与载体中的A蛋白基因融合。重组质粒转化大肠杆菌Top10 ,菌落在 36 5nm紫外光下呈现明亮的绿色荧光。本研究成功构建并表达了GFP与蛋白A的重组质粒 ,为开展GFP的研究及应用提供了生物工程工具的前体。     

cGMP-dependent protein kinases

Cyclic GMP is common second messenger in a plethora of processes. Its major intracellular receptors are the cGMP-dependent protein kinases (PKGs). In this minireview we summarise the main results of studies on structure and physiological role of PKGs.     

Cats are able to adapt protein oxidation to protein intake provided their requirement for dietary protein is met

Cats require more dietary protein than noncarnivorous species. Earlier work showed that cats lack the ability to regulate hepatic urea cycle enzymes in response to dietary protein concentration. We thus hypothesized that cats are unable to fully adapt protein oxidation to protein intake, particularly at low-protein concentrations. We used indirect respiration calorimetry to assess cats' ability to adapt substrate oxidation to diets containing different concentrations of protein, including 1 below their protein requirement. Nine cats (5 males and 4 females; 2.7 +/- 0.5 y; 4.49 +/- 0.19 kg) consumed each of 4 semipurified diets containing 7.5% [low protein (LP(3))], 14.2% [adequate protein (AP)], 27.1% [moderate protein (MP)], and 49.6% [high protein (HP)] of metabolizable energy from protein in a modified crossover design, beginning with the MP diet and then consuming the remaining diets in random order. After adaptation to each diet, cats completed a 5-d nitrogen balance trial and at least 2 12-h indirect calorimetry measurements. There was a significant effect of diet on protein oxidation (P < 0.0001), which measured 10.4 +/- 0.5, 14.1 +/- 1.0, 25.0 +/- 1.7, and 53.2 +/- 1.7% of total energy expenditure for the LP, AP, M,P and HP diets, respectively. The ratio of protein oxidation:protein intake was higher with the LP diet (1.39 +/- 0.07) than the other 3 diets (AP, 1.00 +/- 0.07; MP, 0.93 +/- 0.06; HP, 1.07 +/- 0.03; P < 0.0001), indicating a net loss of protein with the LP diet. Thus, cats are able to adapt protein oxidation to a wide range of dietary protein concentrations, provided their minimum protein requirement is met.     

Relationship between protein intake and hepatic protein synthesis in rats.

Protein synthesis has been measured in vitro in postmitochondrial extracts from livers of rats fed levels of casein ranging from 0 to 40% by weight. The maximal capacity for protein synthesis per milligram of RNA, measured with each amino acid added at 250 mumol/L, was 40-60% higher in rats fed a protein-free diet than in those fed 6 or 15% casein. Our results suggest that the livers of rats fed a protein-free diet are primed for the synthesis of tissue proteins and, given an adequate supply of amino acids, the rate of protein synthesis would be as high as or higher than the rate in protein-replete animals. When amino acids were added to the in vitro system at concentrations found in plasma of rats fed 0, 6, 15 or 40% casein the rate of protein synthesis increased by three- to fourfold over this range, with the highest rate observed for the 15% dietary casein level. We conclude that when protein intake is below the requirement level, the rate of liver protein synthesis may be limited by amino acid supply, by the capacity of the system for protein synthesis or by both.     

Dietary protein, endurance exercise, and human skeletal-muscle protein turnover

PURPOSE OF REVIEW: Whereas diet and exercise have been shown to influence whole-body protein utilization, little is known about the impact of these factors on skeletal-muscle protein turnover. We highlight the role of dietary protein in modulating skeletal-muscle protein turnover in response to endurance exercise. Effects of endurance exercise on skeletal-muscle protein metabolism are presented and the influence of habitual protein intake on exercise-related protein responses is discussed. RECENT FINDINGS: Skeletal-muscle protein turnover increases in response to endurance exercise training and following a single endurance exercise bout. Nutritional supplementation postexercise favorably affects skeletal-muscle protein synthesis and demonstrates amino acid availability as pivotal to the skeletal-muscle synthetic response following exercise. The level of habitual protein intake influences postexercise skeletal-muscle protein turnover. SUMMARY: Dietary protein and exercise are powerful stimuli affecting protein turnover. Since variation in habitual protein intake influences skeletal-muscle protein turnover postexercise, investigations are needed to determine what role protein intake has in regulating skeletal-muscle protein metabolism. Long-term, well controlled diet and exercise intervention studies are essential for clarification of the relation between protein intake, endurance exercise, and skeletal-muscle protein turnover. Studies designed to characterize this relationship should be attentive to habitual macronutrient and energy intakes.     

The SARS-CoV nucleocapsid protein: a protein with multifarious activities.

Ever since the discovery of SARS-CoV in the year 2003, numerous researchers around the world have been working relentlessly to understand the biology of this virus. As in other coronaviruses, nucleocapsid (N) is one of the most crucial structural components of the SARS-CoV. Hence major attention has been focused on characterization of this protein. Independent studies conducted by several laboratories have elucidated significant insight into the primary function of this protein, which is to encapsidate the viral genome. In addition, many reports also suggest that this protein interferes with different cellular pathways, thus implying it to be a key regulatory component of the virus too. In the first part of this review, we will discuss these different properties of the N-protein in a consolidated manner. Further, this protein has also been proposed to be an efficient diagnostic tool and a candidate vaccine against the SARS-CoV. Hence, towards the end of this article, we will discuss some recent progress regarding the possible clinically relevant use of the N-protein.     

Soy protein in relation to human protein and amino acid nutrition

The nutritional value of processed soy protein (isolated soy proteins and soy-protein concentrates) in human protein and amino acid nutrition is evaluated on the basis of a review of studies of growth and nitrogen balance in infants, children, adolescents, and adults. Findings show that well-processed soy-protein isolates and soy-protein concentrates can serve as the major, or even sole, source of protein intake and that their protein value is essentially equivalent to that of food proteins of animal origin. The importance of the sulfur amino acid content of soy protein for practical human nutrition is also examined from nitrogen-balance data. Under conditions of an anticipated normal usage of soy protein, methionine supplementation is not only unnecessary but may even be undesirable for young children and adults. However, for newborns, the data suggest that modest supplementation of soy-based formulas with methionine may be beneficial. Soy proteins have also been found to be of good quality to include in hypocaloric diets for weight reduction in obese subjects. Finally, the data indicate that soy proteins are well-tolerated and of good acceptability.