Dental Biochemistry 2 – (Lec. 10,11)

Biochemistry of Teeth,

Saliva and Dental Caries

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Saliva

• Is the biological fluid, which bathes the oral cavity.

• It is a complex fluid produced by a number of specialized glands which discharge into the oral cavity.

• Saliva contains electrolytes and proteins.

• Electrolytes are minerals in your blood and other body fluids that carry an electric charge.

• The total volume of saliva produced each day in adults is 500 to 1500 ml.

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Functions of Saliva

i. Antibacterial and antifungal action

ii. Buffering

iii. Digestion

iv. Mineralization

v. Lubrication.

vi. Many salivary components do multiple jobs. For example, amylase in addition to being an enzyme also inhibits precipitation of calcium salts.

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Composition of Saliva • The parotid glands produce serous secretions only,

devoid of mucin.

• On the other hand, the submandibular and sublingual glands secrete both serous and mucinous secretions.

• The viscosity of the submandibular saliva usually decreases with increasing flow rate.

• Salivary secretion is stimulated by smell and taste.

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Characteristics of mixed saliva • Volume 500-1500 ml I day • Rate of flow 0.1--0.25 mil min • pH 5.6-7.2 (mean 6.5) • Water content 97-99.5% • Total protein 01--0.6 g/dl • Mucin 0.27 g I dl • Glucose 10-20 mg/dl • Potassium 10-40 mMoI/L • Sodium 2-50 mMoI/L • Calcium 1-2.5 mMoI/L • Magnesium 0.2--0.6 mMoI/L • Phosphate 2-22 mMoI/L • Chloride 5-50 mMollL • Total lipid 20 mg/dl • Cholesterol 7.5 mg/dl

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Inorganic Components

Saliva contains the usual electrolytes of the body fluids, the principal ions being sodium. potassium, chloride and bicarbonate.

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Organic Components • i. Major carbohydrate in saliva is glucose (10-20

mg/dl)

• ii. Almost all the organic compounds of plasma. such as hormones, immunoglobulins and enzymes may be detected in saliva in trace amounts.

• Iii. The total protein concentration in saliva is very little and is less than 1% of that In plasma. Important proteins of saliva include, mucin, statherins, histatins, lysozyme, proline rich proteins (PRPs), carbonic anhydrase, lingual lipase, amylase, peroxidase lactoferrin and immunoglobulin A (lgA).

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Mucins • They constitute the major proteins of the saliva.

• The salivary mucins exist in two forms; MG1 and MG2. Both are glycoproteins.

• They contain negatively charged groups, such as sialic acid and sulfate.

• They are hydrophilic and trap water resulting in high elasticity.

• The oligosaccharide residues bind to bacterial proteins preventing them from adhering to soft tissue and enamel.

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Functions of mucin Tissue coating

• Protective coating about hard and soft tissues

• Primary role in formation of acquired pellicle

• Concentrates antimicrobial molecules at mucosal interface

Lubrication • Align themselves with direction of flow increases

lubricating qualities

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Salivary Enzymes

• The main enzymes present in saliva are the amylase, lingual lipase, carbonic anhydrase and peroxidases.

• Saliva supplies enzymes for digestion.

• These enzymes and other proteins including saliva specific glycoproteins, are synthesized by the acinar cells.

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Amylase • The major salivary enzyme is alpha amylase.

• The amylase acts on carbohydrates. It cleaves the alpha-1,4-glycosidic bonds of starch. The products are small quantities of maltose (disaccharide) and smaller sized polysaccharides.

• The optimum pH of salivary amylase is 6. However, its action is short lived as the food passes into stomach and the enzyme becomes Inactive at the highly acidic pH of the gastric lumen.

• The parotid gland secretes most of the amylase.

• When there is any obstruction to the salivary ducts or inflammation of the glands (as in mumps), the salivary amylase passes into the blood and elevates the level of serum amylase.

• Amylase also shows weak antibacterial properties as well as buffering property. 11

Other Enzymes

• i. Lingual lipase acts on triglycerides and is important in the digestion of milk fat in infants.

• Ii. Carbonic anhydrase is responsible for the buffering action of saliva.

• iii. Peroxidases assist in the bactericidal function.

• iv. Lysozyme in saliva has antimicrobial action. The bactericidal effect is by breaking down the muramic acid present in bacterial cell walls.

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Other Proteins • I. Immunoglobulin A (lgA) is the antibodies present

in body secretions. It may be effective against cariogenic bacteria. IgA levels are found to be low in some persons with dental caries

• ii. Lactoferrin chelates the iron.

• iii. Saliva also contains a group of histidine rich proteins with antifungal activity.

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• iv. Statherins are proteins that keep the supersaturated calcium phosphate in the ductal saliva from crystallizing.

• The supersaturated calcium phosphate is necessary for the maintenance of enamel integrity.

• Statherins bind calcium and prevent precipitation of calcium phosphate. So the probability of formation of dental calculus is reduced.

• The statherins also help in lubrication.

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v. The Proline Rich Protein (PRP) contains a large number of proline residues (40% or more).

• They also reduce precipitation of calcium phosphate.

• PRPs also help in the formation of the enamel pellicle.

• This reduces the bacterial attacks

• Slows down the loss of calcium and phosphate ions from the teeth.

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COMPOSITION OF TEETH • During the formation of teeth, there is a close

association of inorganic (mineral) crystal material, and organic fibrous (polymer) structures, both components playing a structural role in the tooth.

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Inorganic Components

• i. The inorganic calcium is deposited along with phosphate as apatite, which is the major form of calcium in all the tooth tissues.

• A small proportion of other crystalline forms of calcium phosphate may also exist in teeth.

• Amorphous (noncrystalline) calcium phosphate may be found in the dentin.

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• ii. The phosphate ions constitute the major component of the ions present in the crystal.

• iii. The arrangement of phosphate ions results in octahedral channels running through the crystal structure.

• iv. Two-thirds of these channels are occupied by calcium ions.

• The remaining third of the channels are occupied by negative fluoride ions. This is called fluoroapatite.

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Trace Elements

• In human enamel, trace elements such as iron, zinc, copper, and manganese are found.

• Iron and zinc accumulate near the surface of the tooth, i.e. in the outer layers of enamel.

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Organic Components

• Collagen • It is the major protein component of calcifying

tissues like bone, dentin and cementum.

• Each polypeptide chain of collagen has about 1000 amino acid residues.

• The structural proteins and apatite of teeth need to be synthesized in an integrated way.

• In teeth the collagen fibrils are suited to the roles of supporting three-dimensional stress, and of orienting and supporting apatite crystals.

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Other Proteins in Teeth

• i. In addition to collagen, the extracellular matrix also contains glycoproteins (GP) and glycosaminoglycans (GAG).

• These proteins are associated with the dentin and basal plate.

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Proteins of Dentin • The extracellular matrix proteins of bone and dentin

are similar consisting of type 1 collagen, acidic glycoproteins and proteoglycans.

• Collagen forms the lattice for mineralization, but non-collagen proteins control initiation and growth of crystals.

• Three major proteins found specifically in dentin but absent in bone are:

• i. Dentin phosphoryn

• ii. Dentin matrix protein

• Iii. Dentin sialoprotein

• These proteins play an important role in control of mineralization.

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Proteins of Enamel • i. Amelogenin is a low molecular weight

extracellular matrix protein.

• It constitutes about 90% of all enamel protein.

• It has hydrophobic residues on the outside.

Function

• The 27 amino acid portion of amelogenin functions as a calcium channel.

• Phosphorylation of a serine residue of the protein opens the calcium channel, Through which calcium ions zoom through and funneled to the mineralization front.

• It also influences the development of cementum. 23

• ii. Mutation of amelogenin gene leads to amelogenesis imperfecta which is an inherited condition characterized by abnormal enamel formation in quantity, growth, maturation and crystallization in amelogenesis imperfecta.

• The genes are present on X and Y chromosomes designated AMELX and AMELY.

• iii. The other proteins found in enamel are ameloblastin, enamelin and tuftelin.

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Mineralization • I. Mineralization is a process by which inorganic

calcium and phosphate are deposited on the organic matrix.

• ii. Osteoblasts synthesize and secrete organic matrix, which is then mineralized.

• iii. Osteoclasts are Involved in bone resorption.

• iv. Alkaline phosphatase is the key enzyme in the process of mineralization. The enzyme liberates phosphate from substrates, so that Ionic concentration (of calcium x phosphate) is increased to supersaturation level, leading to deposition of apatite.

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DENTAL CARIES

• Caries is a Latin term, meaning "decay".

• There is local destruction of tooth tissues with demineralization.

• Alternative terms are dental cavities or tooth decay.

• In the pits and fissures of premolar and molar teeth, bacterial fermentation of residual food leads to acid production.

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Microbiological Organisms Cause Dental Caries

• The development of caries lesion requires the presence of the bacteria Streptococcus mutans.

• This is generally seen in the oral mucosa and in dental plaque.

• When there is a decrease in saliva flow, the pH of the plaque drops, allowing the acid tolerant bacteria like S. mutans to proliferate.

• S. mutans forms dextran and causes a sticky plaque, trapping bacteria, calcium and phosphate ions.

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Sucrose and Caries

• ii. Sucrose is a low-molecular-weight disaccharide that can be rapidly metabolized by the plaque flora.

• iii. Sucrose fermentation produces lactic acid with consequent drop in the pH, to 5.0 or lower, at the Point of interface between plaque and enamel.

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Other Causes of Dental Caries

• Even though sugars and poor oral hygiene are the major causes of caries.

• In adults chewing of tobacco and exposure to lead, cadmium (metals that can replace calcium) are implicated in the genesis of caries.

• Iodine is found to be able to penetrate enamel, dental pulp and periodontal tissues.

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Prevention of Caries • i. Ideally, oral hygiene is the best way to prevent

caries. This consists of proper brushing at least twice a day and regular dental examination and cleaning, every 6 months.

• ii. However, frequent eating also increases the chances of developing caries, since it keeps the plaque pH low for longer periods. Hence. the Importance of proper cleaning and removing food debris after consumption of food.

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• iii. High molecular weight starch and proteins are not well-utilized by the bacteria. So, milk, fresh fruits and vegetables are not cariogenic.

• iv. Dietary factors that protect teeth against caries are fluoride and sugar free salivary stimulants.

• v. An important concept about treatment of caries is that the destroyed tooth will not regenerate. The aim of treatment is thus to prevent caries or to arrest the progression of caries.

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Fluoride is Useful to Prevent Caries • i. Intake of 2·4 microgram fluoride per day leads to

decrease in the incidence of dental caries.

• Several possible mechanisms are postulated, which include:

a. Effect on hard tissues to modulate mineralization, demineralization and re-mineralization.

b. Effect of cariogenic bacteria by altering their metabolism.

c. Effect on soft tissues to modify the development of teeth.

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Fluorosis is More Dangerous Than Caries

• i. Fluoride level more than 2 ppm (2 mg/l) will cause chronic intestinal upset, gastroenteritis, loss of appetite and loss of weight.

• ii. Levels more than 5 ppm cause mottling of enamel, stratification and discoloration of teeth.

• iii. A level more than 20 ppm is toxic, leading to alternate areas of osteoporosis and osteosclerosis, with brittle bones. This is called fluorosis.

• iv. Ingested fluoride accumulates in bones. It is a cumulative toxin. 34

• v. In fluorosis, blood concentration of fluoride Increases to 50 microgram/100 ml; whereas normal value is 4 microgram/100 ml.

• vi. Fluorosis is characterized by joint defects. Due to increased breakdown of bone matrix, excretion of hydroxyproline in urine is enhanced.

• N. B. • Fluorinated toothpaste contains 3,000 ppm of

fluoride. Even ordinary toothpaste contains fluoride about 700 ppm.

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Prevention of Fluorosis

• Provide fluoride free water,

• Supplementation of vitamin C

• Avoiding fluoride containing toothpaste.

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