Nutritional Biochemistry of the Vitamins and Secondary Metabolites (2016-2)

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Professor D.Sci. Judit Kosáry

Nutritional biochemistry of the vitamins and secondary metabolites (2016-2)

(Lectures for students with advanced knowledge of biochemistry and food chemistry)

The lectures present the known biochemical functions of the vitamins and the possible explanations for the effects of their deficiency or excess. The biosynthesis of vitamins and secondary metabolites used in food industry is also discussed. All of vitamins and other agents are detailed in point of their:

• biosynthesis,
• metabolism,
• metabolic functions,
• deficiency and excess,
• nutritional status,
• uses in food industry,
• pharmacological uses.

Topics:

1. Definition and nomenclature of vitamins,their nutritional status and non-nutritional uses.

2. Precursors of reagents for biochemical reactions (molecules with coenzyme function)(water-soluble vitamins):

a) Precursors of coenzymes of oxydoreductases : niacin, riboflavin, ascorbic acid.

b) Precursors of coenzymes of transferases:

1. C1 transfer: biotin, folic acid, cyanocobalamin,
2. C2 transfer: thiamin, pantothenic acid,
3. transfer of other groups: pyridoxine.

c) Compounds of doubtful vitamin status: taurine, carnitine, choline, inositol.

3. Vitamins of other functions – vitamin lipids (fat-soluble vitamins):

1. retinol and -carotene,
2. cholecalciferol and its vitamers,
3. tocopherols,
4. phylloquinone and its vitamers,

4. Definition and types of secondary metabolites. Secondary metabolites used in food industry (e.g. flavour agents of spices, pigments, antioxidants, etc.).

The lectures also present information about the biochemical functions and the metabolism of some important types of secondary metabolites, which are of different molecules of the living organisms, andthey are more or less needed for their functioning. Secondary metabolites are synthesized from the different intermediates of biomolecules. The presented types of secondary metabolites are coenzymes, regulating (e.g. hormones), attracting (e.g. the sweet sucrose, the fruit esters as scent agents etc.) and repelling agents (e.g. alkaloids and toxins).

Literature: Bender, D.A.: Nutritional biochemistry of vitamins. Cambridge University Press Cambridge New York Port Chester Melbourne Sydney 1992; Luckner, M.: Secondary metabolism in microorganisms, plants and animals Springer Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong 1990.

Nutritional biochemistry of the vitamins and secondary metabolites

Professor D.Sci. Judit Kosáry

The vitamins are a disparate group of organic compounds whose only common feature is that they are essential (cannot be synthesized inside) and required in small amount for the normal functioning of higher animals and the human body, therefore they must be provided in nutrition. These compounds can be synthesized by plants and – some of them – by animals from intermediates of their primary metabolic pathways. According to their role vitamins have diverse functions in the human metabolism. Polar (“water-soluble”) vitamins are precursors of reagents so-called coenzymes in metabolic organic chemical reactions. The water-soluble vitamins are vitamin C and a series known as the vitamin B complex. Nonpolar (apolar) (“lipid-soluble or fat-soluble”) vitamins can be hormones, modulators or regulators and non-specific antioxidants in human metabolism.

In contrast with primary metabolism that is practically identical in all types of living organisms, secondary metabolism is a collection of a wide variety of biochemical pathways characteristic for only a few species of organisms and their distribution is different in specialized cells. Compounds formed in these reactions called secondary products. According to their biosynthetic pathways vitamins can be considered secondary metabolits but with a view to their formation in cell-division (cytogenesis) vitamins can be classified as primary metabolits.

In the course Nutritional biochemistry of the vitamins and secondary metabolites the vitamins are discussed in topics: biosynthesis; metabolism; metabolic functions; explain the effects of deficiency and excess; nutritional status; uses in food industry; pharmacological uses and scientific basis for Recommended Intakes. Topics for secondary metabolites used in food industry: flavour agents, pigments, antioxidants, etc.

1. Definition and nomenclature of vitamins. Their nutritional status and non-nutritional uses.

Essential compounds

A compound, that the human body cannot synthesize them from other compounds at the level needed for normal growth, is called an essential compound. These materials must be obtained from food. The essential amino acids (Val, Leu, Ile, Phe, Lys, Thr, Trp, Met, Arg, His) are needed in a large quantity. Of other essential compounds we only need small quantities e.g. vitamins).

Definition of vitamins

The vitamins are a disparate group of organic compounds whose only common feature is that they are essential (cannot be synthesized inside) and required in small amount for the normal functioning of higher animals and the human body, therefore they must be provided in nutrition. These molecules serve nearly the same roles in all forms of life, but higher animals (and human body) have lost the ability to synthesize them. The original definition of vitamins based on their dietary essential, but now it is partly a historical category. Later it was proved that some of vitamins could be synthesized in the body (e.g. tocopherol and niacin). Nevertheless, for historical reasons these compounds are classified as vitamins.

At the beginning of the twentieth century the history of vitamins started by feeding experiments of Hopkins, who found that animals fed by known and characterised nutrients as fats, proteins, carbohydrates and mineral salt failed to grow without addition a small amount of milk. Hopkins suggested that milk contained ‘accessory growth factors’. The characteristic functional group of the first of ‘accesory food factors” isolated was an amine (thiamin). Therefore these kind of food factors were called by Funk (1912) ‘vitamin’ from the combination of the Latin expression of the word ‘life’ (vita) and the name of the functional group (amine). Later it came to light that the presence of an amine group is not essential forl of vitamins, besides the structures and functional groups of vitamins are highly various but the name ‘vitamin’ is preserved.

Nomenclature of vitamins

At first vitamins had an apparently illoginal system of accepted trivial nomenclature then after identification they got chemical names and short ‘accepted’ names, as well. The basis of the trivial system was on one hand the polar-apolar properties of the vitamin (water-soluble and fat-soluble) and the other hand the order (sequence) of isolation according to ABC. For example at first the name of all of water-soluble vitamins were planned as ‘vitamin B’ depending on the time of isolation e.g. B1, B2 or Bn. Parallel and false identifications made the system confused. In addition there were different chemical compounds with the same biological activity. Now it is known that they are different intermediers of the final active compound therefore they are called vitamers, e.g. retinol and -carotene or nicotinic acid and nicotinamide (niacin) or cholecalciferol and ergocalciferol or phylloquinone and menaquinone.

Table 1. Alphabetical name; accepted name; principal fuctions and deficiency diseases of vitamins

(On the basis of Nutritional biochemistry of the vitamins by D.A. Bender)

Table 2. Alphabetical name; accepted name; name of biologically active derivative and metabolic function

(On the basis of Nutritional biochemistry of the vitamins by D.A. Bender)

Table 3. Alphabetical name of parallel and false vitamin identifications

(On the basis of Nutritional biochemistry of the vitamins by D.A. Bender)

Nutritional status of vitamins – Determination of essentiality

Before declaring a compound as a vitamin it must be shown to be a dietary essential. That means its elimination from the diet results in a more or less clearly defined deficiency disease that can be cured or prevented by adding of this compound to the diet. Only the fact of a pharmacological action or curing a disease is a necessary condition but not a sufficient evidence to classify a compound as a vitamin.

Requirements and recommendations

As vitamins are present in foods and in body fluids or tissues in very small amounts (of the order mol, nmol, even pmol/kg), furthermore they can be exist in multiple chemical forms (sometimes in biologically unavailable forms), their analysis and the estimation of the useable vitamin content demand a combination of several chemical and biological methods. With the modern techniques – e.g. radio-ligand binding assays (radio-immunassay) and hplc techniques – individual chemical forms of most of the vitamins can be determined with great precision and specificity. Microbiological and biological assays can be essential to determine the relative biological activity of different vitamers.

There are several official recommendations for vitamin consumption. On the basis of population examined recommended intakes are various in different countries. All around the world accepted lists are Recommended Dietary Allowance (RDA) for an adult man aged between 25-50 for a day from the 1989 US Tables (National Research Council/National Academy of Sciences, 1989) and Reference Nutrient Intake (RNI) for an adult man aged between 19-50 for a day from the 1991 UK Tables (Department of Health/Ministry of Agriculture, Fisheries and Food, 1991).

Table 4. Recommended intakes of vitamins

(On the basis of Nutritional biochemistry of the vitamins by D.A. Bender)

Non-nutritional uses of vitamins

Recommended intakes of vitamins for people the maitenance of normal health and metabolic integrity and the prevention of deficiency. There are suggestions about the benefit of vitamins (and other nutritional supplements) intake higher than nutritional requirements. In some cases no scientific foundations exist and these hypotheses are based only on their pharmacological action in relatively high intake and speculations. But there are evidences from medical and laboratory studies that some vitamins may have health benefit in relatively high doses. For example folic acid supplements in pregnancy is a protective factor against tube defects and higher intakes of antioxidants (L-ascorbic acid, tocopherols and -carotene) can reduce probability of cancer and cardiovascular diseases. Nevertheless, several vitamins are acutely or chronically toxic in high excess (e.g. retinol, cholecalciferol, niacin and pyridoxin).

Vitamin deficiency (avitaminosis and hypovitaminosis)

Vitamin deficiency is a lack of one or more vitamins in humans (or other living organisms).In the case of hypovitaminosis the level of a vitamin is lower than the recommended level. In the case of hypervitaminosis the level of a vitamin is higher than recommended level. There are five vitamins those are associated with a pandemic deficiency disease: niacin (vitamin B3) – pellagra, ascorbic acid (vitamin C) – scurvy, aneurine (vitamin B1) – beriberi, calciferol (vitamin D) – rickets, retinol (vitamin A) – night blindness.

Iatrogenic hypovitaminosis

There are drugs with antivitamin actions causing called drug-induced malnutrition. For example folics antagonists are used in cancer chemotherapy and some of antischizophrenic agents are riboflavin antagonists. For the normal health smoking persons need more of different vitamins than non-smoking ones.

The solubility of vitamins

The molecules with apolar character (they can not form no H-bonds with water in most of the cases because of their non-polarized bonds) are not soluble in water (fat-soluble molecules). The molecules with polar character (they have polarized bonds e.g. alcohols therefore they can form H-bonds with water molecules) can be soluble in water (water-soluble molecules).

Vitamins are classified as either water-soluble or fat-soluble. In humans there are 13 vitamins. Ordinary four vitamins (A, D, E and K) are labelled as fat-soluble vitamins but sometimes vitamins F are mentioned among them. The water-soluble vitamins are the different vitamins B, vitamin H and vitamin C. Water-soluble vitamins dissolve easily in water, and in general, are readily excreted from the body, to the degree that urinary output is a strong predictor of vitamin consumption. Because they are not readily stored, consistent daily intake is important. Many types of water-soluble vitamins are synthesized by bacteria. Water-soluble vitamins are coenzymes or the starting materials of coenzymes (they are the reagents in the biochemical reactions). Fat-soluble vitamins are absorbed through the intestinal tract with the help of lipids (fats). Because they are more likely to accumulate in the body, they are more likely to lead to hypervitaminosis than are water-soluble vitamins.

Essential polar and apolar characters of simple functional groups

Fat-soluble vitamins

There are two types of lipid (apolar – fatty soluble) biomolecules. Simple lipids cannot be hydrolyzed by sodium hydroxide and complex lipids can be hydrolyzed by sodium hydroxide.Fat-soluble vitamins can be stored in the apolar part of cell membranes, sometimes this can cause hypervitaminosis.

The two main types of simple lipids are the fatty acids and the terpenes. Fatty acids (C16 and C18) are building blocks of complex lipids (neutral triglycerides and phospholipids). Saturated fatty acids are palmitate (CH3(CH2)14–COOH) and stearate (CH3(CH2)16–COOH). Unsaturated fatty acids are the unsaturated versions of stearate (C18): oleate, linoleate and linolenate. The essential linoleate (-6-fatty acid) and linolenate (-3-fatty acid) are known as PUFA (polyunsaturated fatty acids) or vitamins F. Polyunsaturates fatty acids can be found in unsaturated oils (oil of fishes, sunflower, flaxseed – linseed, etc.). Essential fatty acids play an important role in the life and death of cardiac cells.