Kidney , enzymes hormonal control

In the kidneys, parathyroid hormone increases 1 -hydroxylation of calcidiol and reduces 24-hydroxylation. This is not the result of de novo enzyme synthesis, but an effect on the activity of the preformed enzymes, mediated by cAMP-dependent protein kinases. In turn, calcitriol has a direct role in the control of parathyroid hormone, acting to repress expression of the gene. In chronic renal failure, there is reduced synthesis of calcitriol, leading to the development of secondary hyperparathyroidism that results in excess mobilization of bone mineral, hypercalcemia, hypercalciuria, hyperphosphaturia, and the development of calcium phosphate renal stones. 

L-PK (Liver, Kidney, RBC s) greatest regulation. Sigmoidal vs. [PEP] [K1 F-1,6 -bis P is a (+) effector to both. Hormonal control operates via phosphorylation to inactivate the enzyme. 

The volume of extracellular fluid is direcdy related to the Na" concentration which is closely controlled by the kidneys. Homeostatic control of Na" concentration depends on the hormone aldosterone. The kidney secretes a proteolytic enzyme, rennin, which is essential in the first of a series of reactions leading to aldosterone. In response to a decrease in plasma volume and Na" concentration, the secretion of rennin stimulates the production of aldosterone resulting in increased sodium retention and increased volume of extracellular fluid (51,55). 

There seems to be no metabolic control exerted on hepatic 25-hydroxylase and so all of the available cholecalciferol is converted. Hydroxylation in the kidney however is an important control point being regulated by PTH, and indirectly therefore by calcium and phosphate concentrations. Stimulation of la-hydroxylase by PTH is via a cyclic AMP (cAMP) -dependent mechanism and longer-term regulation of the activity of this enzyme is via induction mediated by other hormones such as oestrogens, cortisol and growth hormone. Typically, the plasma concentration of 1,25 dihydroxy vitamin D is in the range 20-60 ng/1, that is approximately 1000-times lower than that of its precursor. 

In the body, insulin is synthesized by j3-cells of Langerhans islets in the pancreas. The rate of formation changes depending on the type of food consnmed, gastrointestinal hormones, and nenronal control. Insnlin circnlating in the body has a biological half-life of about 5 min. It is qnickly broken down by enzymes and is removed from the blood by the liver or kidneys. 

The other major role of selenium is in the production of the thyroid hormones (see p. 127), for which it is a component of the enzyme type I iodothyronine deiod-inase (IDi), which convertsT4 to the physiologically activeT3.AVhen there is a deficiency of selenium the ratio ofT4 T3 increases. The enzyme is found primarily in the liver and kidney and not in the thyroid of farm animals. Type II iodothyronine deio-dinase (ID2) does not contain selenium and also converts T4 toT3, but as it is imder feedback control from T4 an increase in the latter, when selenium is deficient, compounds the problem. The major enzyme in ruminants is ID and in non-ruminants ID2. A third selenium-containing enzyme, ID3, has been found in the placenta. ID is particularly important in the brown adipose tissue of newborn ruminants and releases T3 for use in other tissues. 

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