Mineral Nutrition Of Livestock, 4th Edition

2Chapter 1were shown to improve growth or health inrats, goats, pigs or poultry reared in highly specialized conditions, without inducing specificabnormalities or being associated with breakdown along metabolic pathways where theyhave specific functions (see Chapters 17 and18); these, and some of their predecessors thatwere labelled ‘newer essential elements’ in theprevious edition of this book (notably fluorine,nickel, tin and vanadium) (Underwood andSuttle, 1999), must be re-examined in the lightof the theory of hormesis (Calabrese andBaldwin, 1988). All animal tissues contain afurther 20–30 mineral elements, mostly insmall and variable concentrations. These areprobably adventitious constituents, arising fromcontact with a chemically diverse environment.(Suzuki et al., 2005). The new millennium hasseen an explosion of activity in this field and anew focus: the signalling mechanisms by whichintracellular needs are communicated and orchestrated. Calcium and superoxide ions and selenocysteine play pivotal roles, with the selenocysteine‘altering our understanding of the genetic code’(Hatfield and Gladyshev, 2002). New journalsdedicated to the subject of ‘proteomics’ havebeen launched, but proteomics has yet to impactthe practical nutrition of livestock and it is largelybeyond the scope of this book.FunctionsComplexityMinerals perform four broad types of functionin animals:The last decade of the 20th century saw theincreased application of molecular biology tostudies of mineral metabolism and function, andthe complex mechanisms by which minerals aresafely transported across cell membranes andincorporated into functional intracellular molecules began to be clarified (O’Dell and Sunde,1997). For potassium alone, 10 different membrane transport mechanisms were identified.Genes controlling the synthesis of key metalloproteins such as metallothionein, selenoenzymessuch as glutathione peroxidases and superoxidedismutases (SODs) were the first to be isolated,and deficiencies of zinc were found to influencethe expression of genes controlling the synthesisof molecules that did not contain zinc (Chesters,1992). The induction of messenger RNA fortransport and storage proteins promised to be asensitive indicator of copper deprivation (Wanget al., 1996) but, with ever-increasing arrays tochoose from, animals are proving to be highlyselective in which genes they switch on and whenthey do it. New functions are being revealed,such as the role of zinc in alkylation reactions (seeChapter 16) and some, such as the activation ofa methionine synthetase by copper (see Chapter11), open up fresh possibilities for one elementto compensate for the lack of another. Two families of zinc transporters have been identified,some of which are needed to activate the zincenzyme alkaline phosphatase (see Chapter 16)1. Structural: minerals can form structuralcomponents of body organs and tissues,exemplified by minerals such as calcium,phosphorus and magnesium; silicon in bonesand teeth; and phosphorus and sulfur in muscle proteins. Minerals such as zinc and phosphorus can also contribute structural stabilityto the molecules and membranes of whichthey are a part.2. Physiological: minerals occur in body fluidsand tissues as electrolytes concerned with themaintenance of osmotic pressure, acid–basebalance, membrane permeability and transmission of nerve impulses. Sodium, potassium,chlorine, calcium and magnesium in the blood,cerebrospinal fluid and gastric juice provideexamples of such functions.3. Catalytic: minerals can act as catalysts inenzyme and endocrine systems, as integral andspecific components of the structure of metalloenzymes and hormones or as activators (coenzymes) within those systems. The number andvariety of metalloenzymes and coenzymesidentified has continued to increase since thelate 1990s. Activities may be anabolic or catabolic, life enhancing (oxidant) or life protecting(antioxidant).4. Regulatory: minerals regulate cell replication and differentiation; for example, calciumions influence signal transduction and selenocysteine influences gene transcription, leadingto its nomination as ‘the 21st amino acid’

The Requirement for Minerals3Table 1.1. Some important metalloenzymes and metalloproteins in livestock.MetalFeCuMnSeZnMetalloenzyme or metalloproteinFunctionHepcidinSuccinate dehydrogenaseHaemoglobinCatalaseCytochrome oxidaseLysyl oxidaseHephaestinCaeruloplasminSuperoxide dismutasePyruvate carboxylaseSuperoxide dismutaseGlycosyl aminotransferasesGlutathione peroxidases (four)Type 1 and 2 deiodinasesSelenocysteineCarbonic anhydraseAlkaline phosphatasePhospholipase A2Iron regulating hormoneOxidation of carbohydratesOxygen transport in bloodProtection against hydrogen peroxide, H2O2Terminal oxidaseLysine oxidationIron absorptionCopper transportDismutation of superoxide radical, O2 Pyruvate metabolismAntioxidant by removing O2 Proteoglycan synthesisRemoval of H2O2 and hydroperoxidesConversion of tetraiodothyronine to triiodothyronineSelenium transport and synthesis of selenoenzymesFormation of carbon dioxideHydrolysis of phosphate estersHydrolysis of phosphatidylcholine(Hatfield and Gladyshev, 2002). The pivotalmetabolic role of thyroxine has been attributedto the influence of triiodothyronine on genetranscription (Bassett et al., 2003).An indication of the wide range and functional importance of metalloproteins is given inTable 1.1.Copper is an essential constituent of the growingnumber of cuproenzymes and cuproproteinswith functions as diverse as electron transfer(as cytochrome oxidases), iron absorption (ashephaestin) and antioxidant defence (CuZnSOD)(see Chapter 11).Functional FormsMultiplicity of FunctionMany functions can be performed simultaneously by the same element in the same animaland many take place in both the plants on whichlivestock depend (e.g. glutathione peroxidase)and the microbes or parasites that infect them(e.g. MnSOD and CuZnSOD). Preoccupationwith the structural functions of calcium and phosphorus in the skeleton initially drew attentionaway from their influence on manifold activitiesin soft tissues. These include the maintenance ofcalcium ion concentrations in extracellular fluidfor the orderly transmission of nerve impulsesand intracellular energy exchanges, which allrely on the making or breaking of high-energyphosphate bonds and cell signalling. Phosphorusis an integral part of regulatory proteins and nucleicacids and thus integral to transmission of thegenetic code by translation and transcription.In metalloenzymes, the metal is firmly attachedto the protein moiety with a fixed number ofmetal atoms per mole of protein and cannot beremoved without loss of enzyme activity. Wheretwo metals are present in the same enzymethey may serve different purposes: in CuZnSOD,the ability of copper to change its valency facilitates dismutation of the superoxide free radical,while zinc stabilizes the molecule. Manganesecan also change valency and thus serves a similar function to copper in MnSOD.In regulatory proteins or peptides that contain more than one atom of a mineral, the precise number and/or position of atoms candetermine function. Thyroxine contains fouratoms of iodine, two attached to an outer ringand two to an inner tyrosine ring (T4). Removalof one atom from the outer ring creates a physiologically active molecule (triiodothyronine), whileremoval of an inner atom creates an inactive

4Chapter 1analogue. Deiodination is accomplished by afamily of three selenium-dependent deiodinases, synthesized from the encoded selenocysteine (Beckett and Arthur, 1994). Lowmetalloenzyme activity and low concentrationsof metalloproteins in particular cells or fluidssometimes accompany and explain specificclinical symptoms of mineral deprivation, butsome serious pathological disorders cannot beexplained in such biochemical terms (Chestersand Arthur, 1988).Cobalt is a unique element in that its functional significance can be accounted for by itspresence at the core of a single large molecule,vitamin B12, with two different functions determined by the side chain that is attached.MetabolismMinerals follow labyrinthine pathways throughthe animal once ingested and Fig. 1.1 gives thebarest of introductions. The digestive processcan enhance or constrain the proportions ofingested minerals that are absorbed from thediet and occasionally change the forms inwhich they are absorbed (e.g. selenium).However, minerals are not broken down intometabolizable forms (i.e. ‘digested’) in the waythat organic dietary components are.Absorption of many minerals is carefully regulated, but some share the same regulator, adivalent metal transporter (Garrick et al.,2003; Bai et al., 2008). Iron and manganeseare delivered to a shared protective bindingprotein, transferrin, within the gut mucosa (seeChapter 13). Minerals are usually transportedfrom the serosal side of the mucosa to the liverin free or bound forms via the portal bloodstream, but they can get ‘stuck’ in the mucosa.From the liver, they are transported by theperipheral bloodstream to be taken up by different organs and tissues at rates determinedby local transporter mechanisms in cell membranes and organelles to meet intracellularneeds: a single insight is given into the intricacies of such movements in the context of zinc(see Chapter 16).Mineral turnover rates vary from tissue totissue, but are generally high in the intestinalmucosa and liver, intermediate in other softtissues and slow in bone, although turnoverrates are influenced by physiological (e.g. lactation) and nutritional (deficient or overloaded)state. Minerals also leave the transport pool bysecretion (e.g. milk, sweat and digestive juices)and excretion (urine): those secreted into thegut prior to sites of absorption may be reabsorbed, and the resultant recycling delays theonset of mineral deprivation, as in the case ofphosphorus secreted in saliva. Using computerprogrammes such as SAAM27, rates of exchangeof minerals between body pools or ‘compartments’ with different turnover rates and betweenthe gut and bloodstream can be measured fromthe rates of change in specific radioactivity inselected pools after a single intravenousradioisotope dose. Compartmental analysis hasthus clarified the changes in calcium accretionand resorption in the skeleton during pregnancy and lactation in sheep (Braithwaite,1983) and two minerals with interrelated pathways, such as calcium and phosphorus, can betracked simultaneously (Fernandez, 1995).Figure 1.1a shows a macro-model representing the dynamics of mineral metabolism for calcium at a single moment in time. Similarintracellular fluxes and sequential events takeplace on a much smaller nano-scale betweenorganelles (Fig. 1.1b) and they often rely on thesame transporters that facilitate absorption (seeChapter 13) (Garcia et al., 2007).Net RequirementsThe functions performed by minerals can onlybe fulfilled if sufficient amounts of the ingestedmineral are absorbed and retained to keeppace with growth, development and reproduction and to replace minerals that are ‘lost’either as products, such as milk or eggs, orinsidiously during the process of living.MaintenanceFinite amounts of al

N. Suttle 2010. Mineral Nutrition of Livestock, 4th Edition (N. Suttle) 1 1 The Requirement for Minerals Early Discoveries All animal and plant tissues contain widely vary-ing amounts and proportions of mineral ele-ments, which largely remain as oxides, carbonates, phosphates and sulfates in the ash after ignition of organic matter. In the .