Studies on amino acid metabolism in Lathyrus sativus Biosynthesis of homoserine and O-oxalylhomoserine

Examination of free amino acid pool in <i>Lathyrus sativus</i> showed a rapid increase of homoserine and O-oxalylhomoserine during germination. Isotopic experiments indicated that aspartic acid was an effective precursor of homoserine in <i>Lathyrus sativus</i> and suggested oxalic acid to be incorporated into O-oxalylhomiaserine as an intact moiety. Similar trends of amino acid metabolism of <i>Lathyrus sativus</i> and of <i>Pisum sativum</i> have been discussed.

Protein Turnover, Ureagenesis and Gluconeogenesis

The major processes discussed below are protein turnover (degradation and synthesis), degradation into urea, or conversion into glucose (gluconeogenesis, Figure 1). Daily protein turnover is a dynamic process characterized by a double flux of amino acids: the amino acids released by endogenous (body) protein breakdown can be reutilized and reconverted to protein synthesis, with very little loss. Daily rates of protein turnover in humans (300 to 400 g per day) are largely in excess of the level of protein intake (50 to 80 g per day). A fast growing rate, as in premature babies or in children recovering from malnutrition, leads to a high protein turnover rate and a high protein and energy requirement. Protein metabolism (synthesis and breakdown) is an energy-requiring process, dependent upon endogenous ATP supply. The contribution made by whole-body protein turnover to the resting metabolic rate is important: it represents about 20 % in adults and more in growing children. Metabolism of proteins cannot be disconnected from that of energy since energy balance influences net protein utilization, and since protein intake has an important effect on postprandial thermogenesis - more important than that of fats or carbohydrates. The metabolic need for amino acids is essentially to maintain stores of endogenous tissue proteins within an appropriate range, allowing protein homeostasis to be maintained. Thanks to a dynamic, free amino acid pool, this demand for amino acids can be continuously supplied. The size of the free amino acid pool remains limited and is regulated within narrow limits. The supply of amino acids to cover physiological needs can be derived from 3 sources: 1. Exogenous proteins that release amino acids after digestion and absorption 2. Tissue protein breakdown during protein turnover 3. De novo synthesis, including amino acids (as well as ammonia) derived from the process of urea salvage, following hydrolysis and microflora metabolism in the hind gut. When protein intake surpasses the physiological needs of amino acids, the excess amino acids are disposed of by three major processes: 1. Increased oxidation, with terminal end products such as CO2 and ammonia 2. Enhanced ureagenesis i. e. synthesis of urea linked to protein oxidation eliminatesthe nitrogen radical 3. Gluconeogenesis, i. e. de novo synthesis of glucose. Most of the amino groups of the excess amino acids are converted into urea through the urea cycle, whereas their carbon skeletons are transformed into other intermediates, mostly glucose. This is one of the mechanisms, essential for life, developed by the body to maintain blood glucose within a narrow range, (i. e. glucose homeostasis). It includes the process of gluconeogenesis, i. e. de novo synthesis of glucose from non-glycogenic precursors; in particular certain specific amino acids (for example, alanine), as well as glycerol (derived from fat breakdown) and lactate (derived from muscles). The gluconeogenetic pathway progressively takes over when the supply of glucose from exogenous or endogenous sources (glycogenolysis) becomes insufficient. This process becomes vital during periods of metabolic stress, such as starvation.

Variations in Folate Metabolism in Whole Body γ-Irradiated Mice

Folate metabolism has been studied in 7 Gy whole body y-irradiated mice. Considerable changes in the formyl and methyl tetrahydrofolate derivatives were observed in the liver at various stages after irradiation. Methyltetrahydrofolate polyglutamate cofactors were reduced to a considerable extent in the irradiated group. Significant levels of oxidized folates such as pteroyl mono-, di-, and tetra- glutamyl forms were found only in the irradiated liver indicative of oxidative changes. Methylenetetrahydrofolate reductase, the enzyme synthesizing methyltetrahydrofolates was impaired 35-40% in irradiated animals possibly leading to an impairment in methyltetrahydrofolate synthesis. Folylpolyglutamate hydrolase which hydrolyzes polyglutamylfolates to simpler forms, showed an immediate spurt in the activity but returned to normal values, 48-72 h after irradiation. An increased lysosomal membrane damage as assessed by higher free-amino acid pool was observed in irradiated mice.