Parathyroid Hormone

Parathyroid hormone (PTH), parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands.

It acts to increase the concentration of calcium (Ca2+) in the blood, whereas calcitonin (a hormone produced by the parafollicular cells of the thyroid gland) acts to decrease calcium concentration.

PTH acts to increase the concentration of calcium in the blood by acting upon the parathyroid hormone 1 receptor (high levels in bone and kidney) and the parathyroid hormone 2 receptor (high levels in the central nervous system, pancreas, testis, and placenta).

Effect of parathyroid hormone in regulation of serum calcium.

Bone -> PTH enhances the release of calcium from the large reservoir contained in the bones. Bone resorption is the normal destruction of bone by osteoclasts, which are indirectly stimulated by PTH forming new osteoclasts, which ultimately enhances bone resorption.

Kidney -> PTH enhances active reabsorption of calcium and magnesium from distal tubules of kidney. As bone is degraded, both calcium and phosphate are released. It also decreases the reabsorption of phosphate, with a net loss in plasma phosphate concentration. When the calcium:phosphate ratio increases, more calcium is free in the circulation.

Intestine -> PTH enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH converts vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca2+ ions) by the intestine via calbindin.

FATTY  ACIDS

Fatty acids consist of a hydrocarbon chain with a carboxylic acid at one end.

• are usually in esterified form as major components of other lipids

• are often complexed in triacylglycerols (TAGs)

• most have an even number of carbon atoms (usually 14 to 24)

• are synthesized by concatenation of C2 units.

• C16 & C18 FAs are the most common FAs in higher plants and animals

• Are either:

—saturated (all C-C bonds are single bonds) or

—unsaturated (with one or more double bonds in the chain)

—monounsaturated (a single double bond)

1.Example of monounsaturated FA: Oleic acid 18:1(9) (the number in unsaturated FA parentheses indicates that the double bond is between carbons 9 & 10)

2. Double bonds are almost all in the cis conformation

—polyunsaturated (more then one double bond)

Polyunsaturated fatty acids contain 2 or more double bonds. They usually occur at every third carbon atom towards the methyl terminus (-CH3 ) of the molecule. Example of polyunsaturated FA: Linoleic acid 18:2(9,12)

• the number of double bonds in FAs varies from 1 to 4 (usually), but in most bacteria it is rarely more than 1

Saturated FAs are highly flexible molecules that can assume a wide range of conformations because there is relatively free rotation about their C-C bonds.

PROPERTIES OF TRIACYLGTYCEROLS

1. Hydrolysis : Triacylglycerols undergo stepwise enzymatic hydrolysis to finally liberate free fatty acids and glycerol.

The process of hydrolysis, catalysed by lipases is important for digestion of fat in the gastrointestinal tract and fat mobilization from the adipose tissues.

2. Saponification : The hydrolysis of triacylglycerols by alkali to produce glycerol and soaps is known as saponification.

3.Rancidity: Rancidity is the term used to represent the deterioration of fats and oils resulting in an unpleasant taste. Fats containing unsaturated fatty acids are more susceptible to rancidity.

Hydrolytic rancidity occurs due to partial hydrolysis of triacylglycerols by bacterial enzymes.

Oxidative rancidity is due to oxidation of unsaturated fatty acids.

This results in the formation of unpleasant products such as dicarboxylic acids, aldehydes, ketones etc.

Antioxidants : The substances which can prevent the occurrence of oxidative rancidity are known as antioxidants.

Trace amounts of antioxidants such as tocopherols  (vitamin E), hydroquinone, gallic acid and c,-naphthol are added to the commercial preparations of fats and oils to prevent rancidity. Propylgallate, butylatedhydroxyanisole (BHA)  and butylated hydroxytoluene (BHT) are the antioxidants used in food preservation.

Lipid peroxidation in vivo: In the living cells, lipids undergo oxidation to produce peroxides and free radicals which can damage the tissue. .

The free radicals are believed to cause inflammatory diseases, ageing, cancer , atherosclerosis etc

Iodine number : lt is defined as the grams (number)  of iodine absorbed by 100 g of fat or oil. lodine number is useful to know the relative

unsaturation of fats, and is directly proportional to the content of unsaturated fatty acids

Determination of iodine number will help to know the degree of adulteration of a given oil

Saponification number : lt is defined as the mg  (number) of KOH required to hydrolyse (saponify) one gram of fat or oiL

Reichert-Meissl (RM)  number: lt is defined as the number of ml 0.1 N KOH required to completely neutralize the soluble volatile fatty acids distilled from 5 g fat. RM number is useful in testing the purity of butter since it contains a good concentration of volatile fatty acids (butyric acid, caproic acid and caprylic acid).

Acid number : lt is defined as the number of mg of KOH required to completely neutralize free fatty acids present in one gram fat or oil. In normal circumstances, refined oils should be free from any free fatty acids.

Glycogen Storage Diseases are genetic enzyme deficiencies associated with excessive glycogen accumulation within cells.

• When an enzyme defect affects mainly glycogen storage in liver, a common symptom is hypoglycemia (low blood glucose), relating to impaired mobilization of glucose for release to the blood during fasting.
• When the defect is in muscle tissue, weakness and difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.

Various type of Glycogen storage disease are

Nomenclature for stereoisomers: D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehydes

For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group.

Most naturally occurring sugars are D isomers.

D & L sugars are mirror images of one another. They have the same name. For example, D-glucose and L-glucose

Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. The number of stereoisomers is 2 ⁿ, where n is the number of asymmetric centers. The six-carbon aldoses have 4 asymmetric centers, and thus 16 stereoisomers (8 D-sugars and 8 L-sugars

An aldehyde can react with an alcohol to form a hemiacetal

Similarly a ketone can react with an alcohol to form a hemiketal

Pentoses and hexoses can cyclize, as the aldehyde or keto group reacts with a hydroxyl on one of the distal carbons

E.g., glucose forms an intra-molecular hemiacetal by reaction of the aldehyde on C1 with the hydroxyl on C5, forming a six-member pyranose ring, named after the compound pyran

The representations of the cyclic sugars below are called Haworth projections.

Fructose can form either: 

• a six-member pyranose ring, by reaction of the C2 keto group with the hydroxyl on C6
• a 5-member furanose ring, by reaction of the C2 keto group with the hydroxyl on C5.

Cyclization of glucose produces a new asymmetric center at C1, with the two stereoisomers called anomers, α & β

Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1 extending either:

• below the ring (α)
• above the ring (β).

Because of the tetrahedral nature of carbon bonds, the cyclic form of pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar