Chemistry and Vegetal Biochemistry, Banat’s University of Agricultural Sciences and Veterinary Medicine, Timisoara, Romania
Received date: February 14, 2012; Accepted date: February 16, 2012; Published date: February 18, 2012
Citation: Butnariu M (2012) Bioavailability of Anti–Radical (antioxidants) Activity Compounds. J Anal Bioanal Tech 3:e101. doi: 10.4172/2155-9872.1000e101
Copyright: © 2012 Butnariu M. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Antioxidants (AO) have an important role in the removal of free radicals (FR). AO substances combine with FR and neutralize them. Once neutralized, they no longer damage living organisms.
AO are not completely absorbed and used by the body. For nutrition principles absorbed through a process of passive diffusion, the share of AO absorbed decreases with the increase of the amount of matrix containing it. AO molecular forms for which there are isomers or molecular bonds under the form of esters are important determinants of bioavailability [1-3].
Bioavailability is the ratio between the amount of active substance and the speed with which it is released and absorbed in the body, where it reaches the action site and acts biologically.
Bioavailability of AO compounds can be described through multilinear regression using descriptors of transport and reactivity. Thus, transport speed depends on: hydrophobicity, polarity, degree of ionization, form and size of molecules, removal speed (metabolism), types of interactions (electronic, steric, orbital controlled-frontier-orbitals), etc. Among the factors responsible for this phenomenon there are a series of factors related to diet (chemical formula, presence of inhibitors or enhancers, possibility of interacting with other components) and to body (enzymatic secretions, duration of digestion, activity of intestinal flora, type of nutrition, health state, etc.) [4,5]. AO bioavailability depends on factors such as individual and nutrition type. To determine it, we measure:
-Absolute bioavailability, which corresponds to the fraction of active substance that reaches blood circuit, in general.
-Relative bioavailability, i.e., the amount of active substance reaching general circulation and the speed of this process.
-Relative-optimal bioavailability, which assesses comparatively two forms, of which one is the reference form with maximum availability.
The matrix (physicochemical properties) influences significantly AO bioavailability. In green vegetables, β–carotene can be found under the form of a protein-pigment complex; while in other fruits and vegetables it is contained in lipid drops. AO behaviour in lipids and foods containing lipids depends on the system. The major problem, in this case, is the oxidability of the lipid system, followed by other substratum factors, such as different micro components. Initiation and propagation speeds–which depend on the type and degree of lipid non-saturation– affect AO activity considerably . The effect of an AO is determined by its availability in a “compartment” (a theoretical area of distribution) to combine with receptors on which it acts to produce a specific effect. The availability of an AO is influenced by the transport of molecules through biological membranes, by the degree of bonding between plasmatic proteins and tissues, by the blood flow at the level of action site, and by the metabolising and elimination of AO. To produce the desired effect, an AO needs to cross cell membranes, i.e. two phospholipid layers. Glycoproteins, lipoproteins, as well as different ion or polar groups on the surface of membranes make up membranes. As a result of the interaction of different endogenous or exogenous substances with the receptors on the biological membranes, they can undergo alterations through alterations of the spatial orientation of the resulting compounds and, consequently, channels/pores measuring up to 8Å at the level of cell membrane and up to 60–80 Å at the level of capillaries, open up. Besides the opening of ion channels, the effect of the interaction mediator-receptor can consist in the mobilisation of enzymes mobilising secondary messengers [7-9].
The potential necessary for the transfer through the membranes depends on different factors.
The crossing of the biological membranes by the AO substances depends on:
-Factors depending on biological membranes, i.e.: lipid content; existence of specialized transport systems; polarisation of the membrane; presence of pores; membrane physico–pathological state.
-Factors depending on the AO substance: molecular mass; chemical structure; ionization constant (pKa); rate; lipo/hydro solubility of AO substance, etc.
-Factors depending on the environment on the surface of the biological membranes: pH; protein bonding; vascularisation; blood flow, etc.
AO substances can cross biological membranes in two ways: passive and specialized transfer.
Ways of passive transfer of AO substances at biological membrane level. At this level, there are two ways of passive transfer: simple diffusion and filtering.
Simple diffusion. This is a way of transfer that does not involve energetic consumption and that is based on the difference of concentration of the substance at the level of the two faces of the membrane, with diffusion taking place in the sense of the concentration gradient. This transfer depends on the size of molecules (small molecules diffuse rapidly) and on substance lip solubility. The more the partition coefficient is in favour of lipophilia, the easier the substances diffuse. Above a certain degree of solubility, the substance remains absorbed at the level of biological membranes; level of ionization (is dependent on the polarity of a molecule) .
Biological membranes are easily crossed by non-ionized molecules, but they cannot be crossed through simple diffusion by the molecules ionized or by the substances bonded to plasmatic proteins. Most AO are weak electrolytes, aqueous solutions containing the mixture of nonionized and ionized molecules in a certain balance (AH–non-ionised molecules; A––ionized molecules):
The possibility of crossing biological membranes by AO substances depends on pKa (it is specific to all substances) and by the pH of the absorption site. We can calculate the percentage of the ionised form depending on pH with the Henderson–Hasselbach equation. For substances of the form of weak acids:
log [AH]/[A-]=pKa–pH, where [A–]/[AH]=10(pH–pKa)
[AH]–molar concentration of the non-ionised form; [A–]–molar concentration of the ionized form for substances of the form of weak bases:
log [BH+]/[B]=pKa–pH, where [BH+]/[B]=10(pH–pKa)
[BH+]–molar concentration of the non-ionized form; [B]–molar concentration of the ionized form.
When pKa=pH, the two forms–ionized/non-ionized are in equal concentrations of 50%.
This shows that, for acid AO substances, an alkaline pH increases the percentage of the ionized form, and for basic AO substances, an acid pH increases the percentage of the ionized form. As a result, the transfer of AO substances through the biological membranes depends on the pH of the biological liquid in which the substance was dissolved. In the stomach, where the pH=1–2, acid AO substances are absorbed, while weak bases are ionized in high percentage and are not absorbable from the stomach. Basic AO substances are absorbed from the small intestine, where pH is from little and in the first portion of the duodenum to low alkaline in the next portion of the small intestine, while weak acids are ionized in high percentage and are not absorbable from this portion of the digestive tract .
Filtering. This is a transfer way that does not involve energetic costs and is valid for small hydro soluble molecules (diameter <8Å) that cross biological membranes at the level of aqueous pores under the form of aqueous solution, the transfer occurring due to the differences of osmotic pressure between the two faces of the biological membranes. Large hydro soluble molecules can cross biological membranes at the level of capillaries, where pore diameter is 60–80 Å .
Types of specialised transfer. In many AO substances, membrane transfer cannot occur as shown previously. For such substances, we need specific transport mechanisms, represented by substances with chemical structures that interact with AO substances, after which they cross the biological membrane together with the carrier substance, thus facilitating membrane penetration. Ionic substances also can be carried transmembranary through active transport. The main transfer ways specialized transmembranary are active diffusion (active transport), facilitated diffusion, pinocytosis, phagocytosis, pair ion transport, and absorption biological membranes.
A high percentage of AO substances can be found in the intravascular area under two forms, i.e. free molecules and plasmatic proteinbound molecules. Plasmatic proteins can bind both AO substances and certain endogenous compounds or compounds from foods [13-15].
The main plasmatic proteins are: albumins, representing about 50% of the total serum proteins; alpha 1 acid glycoprotein; lipoproteins, etc. Plasmatic albumins have several bonding sites, with sites for fatty acids and bilirubin, and for AO substances with acid character and strongly ionized at a plasmatic pH 7.4. Bonding AO substances to albumin is achieved through ionic bonds. AO substances with basic character and ionized ones have several bonding sites, but fixing them on plasmatic proteins is achieved through weaker bonds such as Wan der waals, hydrogen bonds, etc. Alpha 1 acid glycoprotein bonds mainly medicines with basic character.
Forming the complex AO substance–plasmatic protein is a reversible process:
where SO–antioxidant substance; P–plasmatic protein; SO–P–complex formed as a result of the interaction of the active substance with the plasmatic protein. There are two constants of the reaction speed in this reaction, i.e.: K1–association constant; K2–dissociation constant. The reaction is reversible, since there is balance between the free AO substance and the plasmatic-bonded one. Bonding to plasmatic proteins is achieved through polar bonds, but there are cases when the AO substance bonds covalently. The bond in the complex resulted from the interaction of the AO substance with the plasmatic protein is characterized by affinity and fixing percentage.
Affinity is expressed through the constant Ka, equal to the ratio between the association constant (K1) and the dissociation constant (K2): Ka=K1/K2.
AO substances bonded to plasmatic proteins are inactive biologically because the fraction bonded plasmatically cannot cross the semi permeable biological membranes. In the blood, AO are free, dissolved in the plasmatic water, bonded to plasmatic proteins (albumins and/or α–1 glycoprotein’s) or to the blood figured elements .
FR occur in certain reactions of oxido-reduction resulting in structural alterations, with, in most cases, alteration of the biological function (it becomes more hydro soluble or it intervenes in another chain of metabolic reactions). The formation of FR brings structural alterations, functional alterations allowing their involvement in multiple reactions.
The human body has natural AO; they make up a complex of enzymes, vitamins, metals, and amino acids functioning in association at two levels: it identifies FR and guides them towards AO molecules that neutralize them and redevelop them again. AO are stable molecules with extra electron or able to receive supplementary electrons. AO constitute the natural system of defence of the body against the damaging effects of the FR. Since FR are not developed exclusively in the environment, but in the human body too, it is essential for us to have a continuous flow of AO for health and longevity, both inner and outer. Reactive oxygen species (ROS) involve the occurrence and evolution of some diseases that can locate in different parts of the human body, generating: diseases of the respiratory system; diseases of the cardio-vascular system; diseases of the digestive system; diseases of the excretory system; diseases of the skin and of the eyes; diseases of the nervous system; endocrine dysfunctions, etc.