Peroxidation of lipids

How peroxidation of lipids happens

Vitamin E dissolves in lipids [Wikimedia]

One of the chem­i­cal and bio­chem­i­cal pro­cess­es that takes place in a liv­ing or­gan­ism is the per­ox­i­da­tion of lipids (it can also be en­coun­tered quite fre­quent­ly in in­dus­try). Usu­al­ly, the type of ox­i­da­tion of fats aris­es as a con­se­quence of free rad­i­cals act­ing on lipid mol­e­cules – par­ti­cles which con­tain in un­paired elec­tron in their struc­ture. Chem­i­cal­ly and bio­chem­i­cal­ly, free rad­i­cals are quite ag­gres­sive sub­stances and cause the de­struc­tion of cells and mol­e­cules. Some­times free rad­i­cals may pre­vent nec­es­sary bio­chem­i­cal re­ac­tions from tak­ing place. Free rad­i­cals arise as a con­se­quence of the bi­o­log­i­cal ox­i­da­tion of mol­e­cules.

Click here for some ex­cit­ing ex­per­i­ments with lipids.

Signs of per­ox­i­da­tion

As this type of ox­i­da­tion is of­ten en­coun­tered in in­dus­try (es­pe­cial­ly the food in­dus­try), it is of­ten de­tect­ed by spe­cial signs – for ex­am­ple an un­pleas­ant smell and ran­cid­i­ty. These signs man­i­fest them­selves when an ex­cess of free rad­i­cals form, which are ca­pa­ble of in­ter­act­ing with fat mol­e­cules.

To slow down the free rad­i­cal ox­i­da­tion process, an­tiox­i­dants are added to prod­ucts – sub­stances that pre­vent the for­ma­tion of free rad­i­cals. Per­ox­i­da­tion starts in dual bonds of un­sat­u­rat­ed fat­ty acids. In cell mem­branes they main­ly take the form of phos­pho­lipids and gly­col­ipids.

Stages of per­ox­i­da­tion

In gen­er­al, the stages of this re­ac­tion (they are equiv­a­lent to a mech­a­nism) are typ­i­cal – they are char­ac­ter­is­tic for any re­ac­tion in which free rad­i­cals take part:

  1. Chain ini­ti­a­tion;

  2. Chain de­vel­op­ment;

  3. Chain branch­ing;

  4. Chain ter­mi­na­tion.

These pro­cess­es may be ex­am­ined from the ex­am­ple of a fat­ty acid or lipid, which has sev­er­al con­ju­gat­ed dou­ble bonds (when dou­ble bonds al­ter­nate with sin­gle ones).

Olive oil consists of liquid fats mixture [Wikimedia]

At the first stage, the hy­drox­yl rad­i­cal is at­tacked by the methy­lene group, lo­cat­ed be­tween the short bonds. The rad­i­cal takes an elec­tron from the group and is then re­duced to wa­ter. The short bond, like the rad­i­cal group, shifts in the mol­e­cule. At the stage of re­ac­tion ini­ti­a­tion, a lipoper­ox­yl rad­i­cal is formed.

In the re­ac­tion of the ob­tained rad­i­cal with neigh­bor­ing mol­e­cules, its neu­tral­iza­tion takes place with the si­mul­ta­ne­ous for­ma­tion of a new rad­i­cal struc­ture (thus mak­ing the free rad­i­cal re­ac­tion a chain re­ac­tion).

In ir­ra­di­a­tion or when hy­droper­ox­ide takes elec­trons from oth­er el­e­ments, the branch­ing of the ini­tial struc­ture is pos­si­ble, which in­creas­es the di­ver­si­ty of prod­ucts of re­ac­tion.

Ter­mi­na­tion of the chain is pos­si­ble in two cas­es – in the re­ac­tion of two rad­i­cals be­tween each oth­er (both of them are neu­tral­ized by the pair­ing off of un­paired elec­trons) or in the ac­tion of an­tiox­i­dants (sub­stances hin­der­ing ox­i­da­tion). For ex­am­ple, to­co­pherol (vi­ta­min E) has this ef­fect. The sub­stance is ca­pa­ble of giv­ing a free rad­i­cal an elec­tron that it lacks, thus mov­ing in­de­pen­dent­ly to a more sta­ble state.

Vitamin E in soft capsules [Wikimedia]

In sim­pli­fied form, the re­ac­tion process is the fol­low­ing:

1. Ini­ti­a­tion:

ROOH + ·OH = ROO· + H₂O;

*2. De­vel­op­ment: *

R· + O₂ = ROO·;

ROO· + RH = ROOH + R·;

3. Ter­mi­na­tion:

ROO· + ROO· = ROOR + O₂;

ROO· + R· = ROOR;

R· + R· = RR.

Prod­ucts of per­ox­i­da­tion

2-hydroxyoleic acid is a fatty acid [Wikimedia]

At the end of ox­i­da­tion of fat­ty acids, 2 main types of prod­ucts form – un­sta­ble di­ene con­ju­gates and per­ox­ides. These are the ini­tial prod­ucts of per­ox­i­da­tion. As one of the prod­ucts has an un­sta­ble struc­ture, sec­ondary and ter­tiary prod­ucts may form – alde­hy­des, di­alde­hy­des and Schiff’s bases re­spec­tive­ly.

Hy­droper­ox­ides can also de­com­pose fur­ther, al­though they are rather sta­ble – among prod­ucts of sec­ondary ox­i­da­tion we may note al­co­hols, alde­hy­des, di­alde­hy­des, ke­tones and epox­ides.

The most ac­tive among prod­ucts of re­ac­tion is mal­onic alde­hyde, as it is ca­pa­ble of re­act­ing with amino groups of many com­pounds which have great bio­chem­i­cal sig­nif­i­cance for the body.

The role of per­ox­i­da­tion

Malonic dialdehyde [Wikimedia]

In the per­ox­i­da­tion of lipids, mal­onic alde­hyde of­ten forms. It is ca­pa­ble of re­act­ing with amino groups of ly­sine, phos­pho­lipids and glu­cosamines, and also N-ter­mi­nal amino acids of pro­teins. Mal­onic alde­hyde “forms” bridges in­side ac­cord­ing mol­e­cules or be­tween them, bond­ing in­di­vid­u­al parts of mol­e­cules or dif­fer­ent mol­e­cules to­geth­er. These bonds change the prop­er­ties of ini­tial sub­stances – for ex­am­ple, the ac­tiv­i­ty of en­zymes change, the struc­tural ca­pa­bil­i­ties of pro­teins drop, the pen­e­tra­bil­i­ty of mem­branes grows from the de­for­ma­tion of re­lease canals, and thus the struc­ture or cell de­forms or even per­ish­es.