1. Physiology of Zinc: General Aspects.- 2. An Introduction to the Biochemistry of Zinc.- 3. Intestinal Absorption of Zinc.- 4. Promoters and Antagonists of Zinc Absorption.- 5. Systemic Transport of Zinc.- 6. Systemic Interactions of Zinc.- 7. Biochemistry of Zinc in Cell Division and Tissue Growth.- 8. Zinc in Cell Division and Tissue Growth: Physiological Aspects.- 9. Biochemical Pathologies of Zinc Deficiency.- 10. Zinc and Iron in Free Radical Pathology and Cellular Control.- 11. Zinc Status and Food Intake.- 12. Zinc and Reproduction: Effects of Deficiency on Foetal and Postnatal Development.- 13. A Note on Zinc and Immunocompetence.- 14. Zinc and Behaviour.- 15. Neurobiology of Zinc.- 16. Zinc in Endocrine Function.- 17. Severe Zinc Deficiency.- 18. Mild Zinc Deficiency in Human Subjects.- 19. Putative Therapeutic Roles for Zinc.- 20. The Diagnosis of Zinc Deficiency.- 21. Human Zinc Requirements.- 22. Dietary Pattern and Zinc Supply.- 23. Zinc Excess.- 24. The Biological Significance of Zinc for Man: Problems and Prospects.

Zinc in Human Biology

Publisher Summary The chapter discusses cerebral protein metabolism, general problems of protein metabolism in other organs and organisms, and some of the recent reviews discussing the areas of importance in cerebral protein metabolism. In the brain, more than in any other organ, the functional metabolism seems to involve changes in the proteins of the organ. The controlling factors of cerebral protein metabolism are of interest, particularly because there is very little regeneration in the central nervous system. Some diseases of the nervous system are apparently caused by the lack of some enzyme proteins normally present. In other diseased states, the presence of abnormal proteins, and structural changes in normal proteins, are found. Also of interest are the reported effects of a number of drugs on cerebral protein metabolism. The control of protein metabolism in all its aspects is the most important problem. It is related to processes of growth, differentiation, degeneration, as well as maintenance of the cells and their protein complement. The growth factor discussed affects one or more of these processes. This factor is specific for a part of the nervous system, but controlling factors of decisive influence may be operative that are specific for other parts of the nervous system, or specific for a cell type in other organs. The knowledge of the mechanism of action of such factors sheds light on the control mechanisms of cellular metabolism.

Protein metabolism of the nervous system.

EARLY studies of insulin shock, particularly those by HIMWICH and his collaborators, showed that profound hypoglycaemia is accompanied by a decreased cerebral uptake of glucose and of oxygen from the blood stream. HIMWICH (1951) reviewed these studies and cited much evidence indicating that the behavioural and electrographic manifestations of hypoglycaemia are related to the reduced supply of glucose to the brain and the consequent decrease in available oxidative energy. More recently, GEIGER (1958) and his coworkers found that the perfused brain of the cat is able to function for more than 1 hr without exogenous glucose if a high perfusion rate is maintained ; this finding suggests that the brain can utilize endogenous non-carbohydrate substrates and that hypoglycaemic symptoms are due in part to the accumulation of toxic breakdown products of these substrates. ABOOD and GEIGER (1955) found decreases in the protein, lipid and nucleic acid contents of the perfused cat brain deprived of glucose and an associated accumulation in the perfusing blood of small quantities of nitrogenous substances, especially GABA, glutathione, glutamate and creatine. On the other hand, SAMSON, DAHL, DAHL and HIMWICH (1959) found that insulin shock did not significantly decrease the cerebral proteins in cats, and DAWSON (1950) made the same observation in rats. A number of other cerebral constituents are known to undergo quantitative changes during hypoglycaemia. These include glycogen, acid-soluble phosphates, and certain of the free amino acids and related compounds. Some of these substances represent possible sources of limited amounts of energy, either as oxidative substrates or as phosphorylated intermediates, while others may occur as breakdown products of proteins, lipids, or other substrates. Pertinent reports are cited in subsequent sections of this paper; in general, these chemical changes have not been studied in temporal relation to the development of electrographic abnormality. We have attempted to follow simultaneously the progression of the electrographic and a number of chemical changes in insulin shock, and their reversal on termination of the hypoglycaemia. E X P E R I M E N T A L The experiments were done on adult male dogs weighing 8-23 kg. Food was withheld for a period of 18-24 hr, after which the cranium was exposed and opened and most of the calvarium was removed, the dura mater remaining intact. Preparation was made for electrographic recording from three cortical areas and for freezing the brain in situ with liquid air. Blood pressure was measured from a cannulated femoral artery in the final stages of the experiment. The details of these procedures have been described (TEWS, CARTER, ROA and STONE, 1963).

Chemical changes in the brain during insulin hypoglycaemia and recovery

THE more closely the brain barrier system is examined, the more its complexity is recognized. I t now appears to be an intricate homeostatic mechanism with metabolitespecific controls that can be influenced, rather than just a passive membrane (see LAJTHA, 1961). Although the uptake of acidic, neutral, and basic amino acids by the brain is impeded when plasma levels are elevated, a rapid exchange of these amino acids occurs between plasma and brain. Such results were obtained with glutamic acid (SCHWERIN, BESSMAN and WAELSCH, 1950; LAJTHA, BERL and WAELSCH, 1959), lysine (LAJTHA, FURST, GERSTEIN and WAELSCH, 1957; LAJTHA, 1958) and leucine (LAJTHA, 1959; LAJTHA and TOTH, 1961). The above findings together with the evidence of competitive inhibition of cerebral tyrosine uptake by other amino acids (CHIRIC~OS, GREENGARD and UDENFRIEND, 1960) and the finding of an increased exchange rate of an amino acid between plasma and brain when the brain level of the amino acid was elevated (LAJTHA and MELA, 1961) pointed to the participation of carrier-mediated rather than passive diffusion processes in the passage of amino acids into and from the brain. This possibility was further underlined by the finding (LAJTHA and TOTH, 1961) of a transport of amino acids from the brain against an elevated plasma level. Further information about the mechanism of passage of cerebral amino acids was hoped to be gained by a study of the egress of amino acids from the brain. In the processes of exit as well as those of uptake, carrier mediated and active transport processes may have a controlling role. This paper presents the results of measurements of the levels of amino acids in the brain at various times after their intracerebral administration.

The brain barrier system. III. The efflux of intracerebrally administered amino acids from the brain.

Publisher Summary This chapter discusses γ-aminobutyric acid (GABA), a substance that has a unique occurrence in the central nervous system (CNS). The chapter focuses on those aspects of the problem in which chemically understandable phenomena are related to normal or abnormal function in the CNS. GABA has inhibitory effects in both invertebrate and vertebrate systems and may play an important role in the regulation of neuronal activity. Although experimental difficulties existed in studying GABA because of the failure of this substance to penetrate the blood–brain barrier readily, the findings that convulsant hydrazides could lower GABA in the brain and that hydroxylamine could increase the content have given potent tools with which to manipulate the intracellular levels of GABA and to study the electrophysiological correlates of the induced changes. Aside from the theoretical importance of the understanding of the role of an inhibitory substance that seems to be formed and located almost entirely in the gray matter of the CNS, some practical aspects of possibilities in the treatment of human disease are apparent. Decreases in brain levels of GABA are associated with the occurrence of paroxysmal discharges, and some types of convulsive disorders in humans might be related to malfunctioning of the GABA system.

Metabolic and neurophysiological roles of gamma-aminobutyric acid.

FLFXNER (1955) has shown that the cytological changes occurring during the development of nervous tissues are associated with concurrent changes in the pattern of enzymic activity. Changes have been reported in the activity of various enzymes in a number of localized regions of the brain (NACHMANSOHN, 1939, 1940; YOUNGSTROM, 1941; H~MWICH and APRXSON, 1955), but little systematic information is available, however, about the enzymes of the hypothalamus. The hypothalamic centres are of interest in view of their role in relation to the chief autonomic and endocrine functions of the body, and since they may also influence the activity of the cerebral cortex (LE GROS CLARK and MEYER, 1950). The vegetative functions of the hypothalamus are particularly important in the 'early life of the animal. The development of biochemical activity in the hypothalamo-neurohypophysial system was studied by HELLW and ZAIMIS (1949), who found a lower concentration of antidiuretic hormone in the neurohypophysis of the newborn than of the adult rat. DAWSON (1953) observed the appearance of neurosecretory substance in the hypothalamus and posterior pituitary of the rat within a few days after birth. In the present investigation the maturation of the hypothalamus of the rat was studied by following the changes with age in the activities of the enzymes phosphomonoesterase I and I1 (nonspecific alkaline and acid phosphatase) and acetylcholinesterase. The values obtained for the hypothalamus were compared with values obtained concurrently for the cerebral cortex and the thalamus. In addition, histochemical techniques were applied to study the localization of the two phosphomonoesterases in the different hypothalamic nuclei. More information was thus obtained about the distribution of these enzymes than could be obtained by biochemical or histochemical methods alone.

Enzymic development and maturation of the hypothalamus.

GAITONDE and RICHTER (1953, 1955) reported that 35S-labelled methionine is rapidly incorporated into the proteins of the rat brain in vivo. By determining the specific radioactivity in the acid-soluble fraction as well as in the isolated proteins of the brain they obtained evidence that, although the total amount of amino acid traversing the blood-brain barrier may be. small, the rate of turnover of amino acids in the proteins of the brain is relatively high. The high metabolic activity of the brain proteins was confirmed in a further investigation in which the blood-brain barrier was by-passed by injecting the labelled amino acid directly into the cerebrospinal fluid (GAITONDE and RICHTER, 1956). It was found that, as BORSOOK et al. (1950) had previously shown for the liver and other organs, the half-life calculated for the mixed brain proteins depended on the time allowed for incorporation, being shorter in expeiiments of less than 60 minutes duration than in experiments of 3 and 5 hours. Calculations based on the uptake of [35S]methionine in uivo during a period of 2MO minutes indicated a mean half-life of the order of 14 days. Since the proteins of the brain are not homogeneous, estimates of the mean rate of amino acid turnover in the mixed proteins of the whole brain are of limited significance. In the present investigation the proteins of the rat brain were fractionated in order to determine the nature of the proteins labelled most rapidly and to study the sequence of labelling in the various protein fractions at different times. Cell fractions containing nuclei, mitochondria, microsomes and soluble proteins were first separated by fractional centrifugation. The proteins in these cell fractions were then further separated into subfractions (A), (B) and (C) by successive extraction with sodium chloride and sodium hydroxide solutions. The results of this study have been described in a preliminary report (CLOUET and RICHTER, 1957): similar studies have recently been reported by KRAVCHINSKY and SILICH (1957) and by FURST, LAJTHA and WAELSCH (1958).

The incorporation of [35S] labelled methionine into the proteins of the rat brain.

THE free amino acid level in the rat brain is second only to that in the kidney, which is the highest in the body (FREDBERG and GREENBERG, 1947). The significance of the high amino acid content of the brain is not yet clear, but experiments with isotopically labelled amino acids have shown that free amino acids are rapidly incorporated into the brain proteins (GAITONDE and RICHTER, 1955, 1956). Two amino acids are present in particularly high concentration in the brain: glutamic acid, which appears to serve a special role, since its level varies with changes in the physiological state (ANSELL and RICHTER, 1954; DAWSON, 1953; VRBA, 1956); and aspartic acid, which is present mainly as the acetyl derivative in rat brain (TALLAN, 1956). The basic amino acids, arginine and histidine, are also of interest in view of the work of WILLIAMS, SCHURR, and ELVWJEM (1950), who reported marked changes in the concentrations of these amino acids in the brain of animals subjected to cold stress and to physical exercise. Although chemical methods are available for estimating arginine and histidine, their application to tissue extracts is dillicult, owing to the low concentrations of these compounds normally present in the tissue and the lack of specificity of the methods. In the present investigation the specificity of the methods was improved by the preliminary chromatographic separation of the basic amino acids on a resin column: this enabled histidine to be separated from histamine and arginine from glycocyamine before their determination in the same tissue sample. Using this method, a study was made of the normal levels of histidine, arginine, and histamine in the rat brain; the effects of anaesthesia, hypoglycaemia, excercise, and cold stress were examined.

D. Richter

Papers

Enzymic development and maturation of the hypothalamus.

The incorporation of [35S] labelled methionine into the proteins of the rat brain.

The free histidine, histamine, and arginine content of the rat brain.