Sterols (Steroids)

 Sterols (Steroids)

Introduction

A steroid is a biologically active organic compound with four rings arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes which alter membrane fluidity; and as signaling molecules. The steroid core structure is typically composed of seventeen carbon atoms, bonded in four fused rings: Examples include the  cholesterol, the sex hormones- estradiol & testosterone and vitamin D₃.

Types

The major types of steroid hormones are-

1. Corticosteroids:

(i)                Glucocorticoids

(ii)             Cortisol, a type of glucocorticoid whose functions include immunosuppression

(iii)           Mineralocorticoids, Aldosterone- a mineralocorticoid which helps regulate blood pressure through water and electrolyte balance.

2. Sex steroids-

(i) Progestogens: Progesterone, which regulates cyclical changes in the  endometrium of the uterus and maintains a pregnancy

 (ii) Androgens: Testosterone, which contributes to the development and maintenance of male secondary sex characteristics

 (iii) Estrogens: Estradiol, which contributes to the development and maintenance of female secondary sex characteristics

3. Other steroids- These include-

(i) Neurosteroids such as DHEA and allopregnanolone

(ii) Aminosteroid neuromuscular blocking agents such as pancuronium bromide

(iii) Secosteroids (open-ring steroids):Vitamin D forms such as  ergocalciferol, cholecalciferol, and calcitriol.

(iv) Cholic acid of bile is a steroid compound.

(v) Cholesterol is a sterol

Chemistry

Steroids are derivatives of phenanthrene and have the parent nucleus perhydrocyclopentanophenan  with 17 carbon atoms consisting in 3 hexagon and one pentagon rings. When the natural steroids have a methyl group at C-13 and at C-10 and possess one or more alcoholic groups, then these secondary alcohols are sterols.

Steroids are found free or in cater forms as cholesterides and can be extracted by organic solvents along with fats.

The steroid core structure is typically composed of seventeen carbon atoms, bonded in four fused rings: three six-member cyclohexane rings (A, B and C) and one five-member cyclopentane  ring (D ring).

Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxy group at position three and a skeleton derived from cholestane. 

Steroids can also be more radically modified, such as by changes to the ring structure, for example, cutting one of the rings. Cutting Ring B produces secosteroids one of which is vitamin D₃.

Properties

Sterols cannot be hydrolyzed by NaOH. Hence, they are not saponifiable. Otherwise, their physical and chemical properties are very similar to those of fats. With fatty acids they form waxes. The various members of this group can be separated generally by fractional distillation.

Distribution

Sterols are widely distributed in both animal and vegetable kingdoms. In the plants they exist chiefly as phytosterols. In the animals they are present in all cells, both in the cell membrane as well as in the cytoplasm. They always remain along with phospholipids. They are found both free sterols as well as sterides. They are more concentrated in certain tissues, as in the nervous tissue, suprarenal cortex and gonads.

Synthesis, destruction and excretion

Sterols can be synthesized in the body from non-sterol sources. The body can destroy sterols only up to some extent. Large quantities are excreted through sebum of the skin secreted by the sebaceous glands. It is also excreted as coprosterol of the feces, and as sex hormones and traces of cholesterol in the urine.