Hormone metabolic functions

Metabolic Functions. The functions of the thyroid hormones and thus of iodine are control of energy transductions (121). These hormones increase oxygen consumption and basal metaboHc rate by accelerating reactions in nearly all cells of the body. A part of this effect is attributed to increase in activity of many enzymes. Additionally, protein synthesis is affected by the thyroid hormones (121,122). 

Most of the physiologic activity of thyroid hormones is from the actions of T3. T4 can be thought of primarily as a prohormone. Eighty percent of needed T3 is derived from the conversion of T4 to T3 in peripheral tissue under the influence of tissue deiodinases. These deiodinases allow end organs to produce the amount of T3 needed to control local metabolic functions. These enzymes also catabolize T3 and T4 to biologically inactive metabolites. Thyroid hormones bind to intracellular receptors and regulate the transcription of various genes. 

Cannabimimetics are also shown to affect reproductive and metabolic functions indirectly by hormonal modulation through the hypothalamic and pituitary regulatory centers. They are found to reduce serum levels of the luteinizing hormone, prolactin, growth hormone, and thyroid-stimulating hormone, and to increase corticotropin (Murphy, 1998). 

The balance between excess and insufficient zinc is important. Zinc deficiency occurs in many species of plants and animals, with severe adverse effects on all stages of growth, development, reproduction, and survival. In humans, zinc deficiency is associated with delayed sexual maturation in adolescent males poor growth in children impaired growth of hair, skin, and bones disrupted Vitamin A metabolism and abnormal taste acuity, hormone metabolism, and immune function. Severe zinc deficiency effects in mammals are usually prevented by diets containing >30 mg Zn/kg DW ration. Zinc deficiency effects are reported in aquatic organisms at nominal concentrations between 0.65 and 6.5 pg Zn/L of medium, and in piscine diets at <15 mg Zn/kg FW ration. Avian diets should contain >25 mg Zn/kg DW ration for prevention of zinc deficiency effects, and <178 mg Zn/kg DW for prevention of marginal sublethal effects. 

Figure 5.2 The Hypothalamic Pituitary Axis The hypothalamus is part of the diencephalon within the brain. The pituitary, although located within the skull, is more correctly a part of the endocrine system than the nervous system. Together, the hypothalamus and pituitary form the interface between the nervous system and endocrine system and exert control over the majority of other hormone secreting organs. Releasing and inhibiting factors permit the hypothalamus to control the pituitary. Pituitary hormones are released into the general circulation, affecting metabolic function throughout the thorax and abdomen.

The thyroid and parathyroid glands are two important endocrine organs that are heavily committed to the biosynthesis of hormones as chemical messengers. The thyroid gland, which surrounds the larynx, has an enormous variety of metabolic functions. It is itself regulated by thyroliberin, which in turn regulates production of thyrotropin (thyroid... 

The Sertoli cells in the testes appear to be the main target of the testicular toxicity. Proposed mechanistic h otheses relate to reduced testicular zinc levels, altered hormonal status, altered metabolic function and altered follicle-stimulating hormone reactivity. 

Thyroid function tests are often altered by somatropin because of increased conversion of T4 to T3, but this is clinically insignificant at low doses (SEDA-21, 453). One child with Prader-Willi syndrome had a fall in serum thyroxine concentration during somatropin therapy and needed thyroxine replacement (33). Hypothyroidism developed in 11 of 46 growth hormone-deficient children treated with somatropin (34). Prior abnormalities in hypothalamic-pituitary function and alterations in thyroid hormone metabolism, probably both, contributed to the high incidence of hypothyroidism, which was similar to that in previous studies. 

Thyroid hormones affect a wide variety of peripheral tissues throughout the individual s life.8,55 In some situations, these hormones exert a direct effect on cellular function (e.g., T4 and T3 appear to increase cellular metabolism by directly increasing oxidative enzyme activity). In other instances, thyroid hormones appear to play a permissive role in facilitating the function of other hormones. For instance, thyroid hormones must be present for growth hormone to function properly. The principal effects of the thyroid hormones are listed below. 

Exogenous insulin is administered to replace normal pancreatic hormone production in type 1 diabetes (IDDM). Exogenous insulin is crucial in maintaining normal glucose levels and proper metabolic function because beta cell function is essentially absent in patients with type 1 diabetes. Without exogenous insulin, the general health of type 1 patients is severely compromised, and they often succumb to the metabolic and neurovascular derangements associated with this disease. 

Ethylene (C2H4) is produced in essentially every part of every seed plant and affects a number of metabolic functions in very small concentrations. It is therefore considered a plant hormone [38]. Cultured plant cells are also known to produce C2H4. 

Selenium This metal is an essential trace element that functions as a component of enzymes involved in antioxidant protection and thyroid hormone metabolism. The existence of a number of selenoproteins has been demonstrated. In several intra- and extracellular glutathione peroxidases and iodothyronine... 

Mammalian nuclear receptors for steroid hormones (SHRs) are of great importance in physiology and medicine, because they control not only developmental pathways but also regulate central physiological and metabolic functions in the adult organism. Steroid hormones and vitamin D are derivatives of cholesterol. Structures of vitamin D3 and of... 

Most of the compounds cited in this introductory section are produced in metabolic processes where the cyclopropane-containing metabolite appears to be the stable end product or secondary product with as yet unobvious metabolic function. However, this is not the case in at least two types of systems, in which cyclopropyl species are key and necessary intermediate structures in high flux metabolic pathways. The first example is the squalene (76) and phytoene (88) biosynthesis where presqualene pyrophosphate (77) and prephytoene pyrophosphate (89) are obligate cyclopropanoid intermediates in the net head-to-head condensations of two farnesyl pyrophosphate (73) or two geranylgeranyl pyrophosphate (66) molecules respectively. The second example is in plant hormone metabolism where C(3) and C(4) of the amino acid methionine are excised as the simple hormone ethylene via intermediacy of 1-aminocyclopropane-l-carboxylic acid (9). Both examples will be discussed in detail in the Section II. 

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