A requirement for several vitamins has been shown: thiamin, riboflavin, folic acid, niacinamide, pantothenic acid, pyridoxine (; ; ; ), biotin () and vitamin B12 (). The following essential amino acids are required to sustain population growth of and : arginine, histidine, lysine, tryptophan, phenylalanine, methionine, threonine, leucine, isoleucine and valine (), although the animals can synthesize limited quantities from labeled precursors (; ; ; ; ). By contrast, alanine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, serine, and tyrosine are not essential nutrients ().
The search for a chemically defined medium that would support sustained population growth of led to the formulation of a mixture, designated EM1, which was based on the amino acid ratios found in (). This medium was later modified by ) and called Maintenance Medium (CbMM). This medium contained a total of 53 components consisting of minerals, glucose, amino acids, vitamins, growth factors and precursors for nucleic acid synthesis (adjusted to pH 5.9 with KOH). Maintenance Medium (CeMM) has the same basal composition, but contains either more glucose or potassium acetate for energy (; ). This basal medium, which can be replaced by 3% soy peptone and 3% yeast extract, must be further supplemented with sterol and a heme source.
In vertebrates, carbon atoms of carbohydrates, lipids and amino acids are usually completely oxidized to CO2 under aerobic conditions. Carbon dioxide is also a major waste product in but, like many other invertebrates and micro-organisms, the worm can also excrete other types of carbon waste. ) and ) found that excretes substantial amounts of glycerol as a major radioactive product when incubated with 2-14C-acetate in complete nutritional medium, but in unsupplemented buffer solution little or no glycerol was produced. The advantage of glycerol excretion in a nutrient-rich environment is not clear. Possibly it is a wasteful means to dispose of superfluous products of metabolism in a nutrient-rich environment, much like synthesis and excretion of amino acids to dispose of end products of nitrogen metabolism. This is consistent with the observation that and are typical opportunistic species that need dense bacterial populations to maintain rapid growth and fecundity. Rapid excretion of glycerol to the external medium is also associated with recovery from hypertonic stress (see ). Like other soil nematodes, is hyperosmotic with its normal environment. The animals accumulate and excrete glycerol as needed to maintain cellular osmotic homeostasis (; ).
Nematodes, including , also possess the glyoxylate cycle enzymes, which convert two acetyl-CoA units into succinate and malate (; ; ; ; ; ). Key enzymes are isocitrate lyase, which cleaves isocitrate to succinate and glyoxylate, and malate synthase, which condenses glyoxylate with acetyl-CoA to form malate. has only one bifunctional glyoxylate cycle protein, which contains separate domains for both enzyme activities on a single polypeptide (). In plants, the glyoxylate cycle operates in specialized peroxisomes, called glyoxysomes. The malate produced in glyoxysomes can be converted to oxaloacetate, completing the cycle. Alternatively, succinate is either transported to the mitochondria where it is converted to malate and thereby consumed in the citric acid cycle, making the glyoxylate cycle an anaplerotic (refilling) process, or transported to the cytosol and further oxidized to oxaloacetate for entry into gluconeogenesis (net production of carbohydrate from triacylglycerols) ().
Ammonia is the main excretory product of nitrogen metabolism, but nitrogen is also excreted in the form of amino acids (; ). ) identified several amino acids as excretion products of axenic populations of after they had been cultured on various radioactive precursors; the concentration of excreted amino acids was greater than the concentration retained in the tissues. ; ) found evidence for a functional urea cycle of low activity, and excretion of urea in .
Mutations that compromise the function of the mitochondrial electron transport chain might be predicted to shorten the lifespan but, remarkably, such mutations frequently confer lifespan extension (). In a systematic RNAi screen for genes conferring longevity, genes related to mitochondrial function were over-represented tenfold (). Long-lived worms, in which mitochondrial function was disturbed by RNAi, displayed low ATP levels and oxygen consumption rates compared to appropriate controls. These metabolic phenotypes confirmed an earlier study in which the function of several subunits of the electron transport chain complexes I, III, IV and V were compromised by RNAi knockdown (). These animals were small and showed slow development and behavior (pharyngeal pumping and defecation). Small size, however, is not a universal hallmark of mitochondrial mutants. Hypometabolic mutants, which contain a defect in an iron-sulphur protein subunit of complex III, were reported to have a normal body size (). Some mitochondrial mutants of the Clock family, such as (a gene involved in ubiquinone synthesis) (; review by ) and (involved in the efficiency and fidelity of mitochondrial protein synthesis), are not hypometabolic (; ). Indeed, in mutants complex I activity remains unaltered (; ; ), although this was challenged by ) who found that complex I activity was decreased by approximately 70% while complex II activity was left intact when compared to wild type.
Various plant metabolites are useful for human life, and the induction and reduction of these metabolites using modern biotechnical technique is of enormous potential important especially in the fields of agriculture and health. describes the biosynthetic pathways of plant metabolites, their function in plants, and some applications for biotechnology. Topics covered include:
Apart from direct physiological measurements, Ins/IGF-mutant metabolism was also studied by analyzing transcriptional profiles of metabolism-related genes. ) found that, in mutants, gluconeogenesis, glyoxylate pathway activity and trehalose biosynthesis were upregulated relative to the appropriate controls ( versus adults). These authors found similar qualitative changes in dauer larvae compared to recovered dauer larvae. Unlike dauer stage animals, TCA-cycle and respiratory chain activities were not downregulated in mutants, supporting the physiological data discussed above (). In addition, in part explanation for the high ATP levels found in these mutants, the mitochondrial F1-ATPase inhibitor protein (IF1), which specifically inhibits the ATPase activity of ATP synthase under anoxic conditions, was found to be upregulated in .
In addition to long-chain fatty acids, ethanol, n-propanol and acetate also promote population growth of in chemically defined medium (). These compounds are utilized as energy sources by , since comparable results were obtained when they were replaced with various carbohydrate supplements. found that glucose and glycogen, followed by trehalose, fructose and sucrose can be utilized as energy sources. reported that is capable of aerobic, but not anaerobic, catabolism of ethanol. Di- and tricarboxylic acids can probably be transported from the extracellular environment into the gut cells and used as a fuel source in the TCA cycle ().
Lipids and amino acids can also be used as energy sources, but they enter the main pathways at different points. has a functional methylmalonyl-CoA epimerase (racemase) that is involved in propionyl-CoA metabolism for the degradation of branched amino acids and odd-chain fatty acids (). Fatty acid moieties of lipids are broken down by -oxidation into acetyl-CoA (which in turn can enter the TCA cycle). -oxidation occurs in the mitochondrial matrix and also yields reduced electron carriers. Peroxisomal -oxidation of long-chain fatty acids is not linked directly to energy metabolism because the reduced electron carrier is directly oxidized by molecular oxygen (yielding hydrogen peroxide). Amino acids can be broken down via distinct pathways and their carbon skeletons can be metabolized in the TCA cycle.
The intermediary metabolic network is well conserved among eukaryotes and the major metabolic pathways found in heterotrophic organisms are also present in (). In the first section of this chapter, we will elaborate on the characteristics of these general pathways in our model. We will subsequently focus on the vital compounds that cannot be synthesized by and hence need to be extracted from the environment. Vitamins, essential amino acids and other related compounds will be discussed in the section about nutritional requirements. At the end of this chapter, a short overview will also be given describing the different metabolic waste products and the storage and production of metabolic energy in .
Major questions that remain unanswered include: How does a mutation affecting mitochondrial function lead to extended lifespan? and What kind of metabolic changes are involved in the process? ) presented plausible biochemical scenarios in partial explanation, ranging from a reduction in overall metabolic rate to activation of compensatory metabolic pathways to combat ROS. For example, mild increases in ROS could evoke hormesis effects resulting in enhanced ROS scavenging. Alternatively, other pathways that produce less ROS, such as fermentation, could be induced (). Although many of these proposed mechanisms are indirectly supported by existing literature, these hypotheses still await direct experimental validation.