7-Dehydrocholesterol (7-DHC) is a compound found within human skin that is structurally similar to cholesterol. This molecule is an intermediate substance the body creates during the synthesis of other compounds. Its presence in the skin positions it at a junction between two separate and important biological functions.
Role in Cholesterol Metabolism
Cholesterol is a waxy substance and a primary component of the membrane surrounding every cell. The body produces it through a multi-step process where enzymes modify various molecules in a sequence. 7-Dehydrocholesterol holds a specific position in this chain as the immediate precursor to cholesterol.
The conversion of 7-dehydrocholesterol into cholesterol is the final step of this pathway. This reaction is facilitated by an enzyme called 7-dehydrocholesterol reductase (DHCR7). The DHCR7 enzyme acts on the 7-dehydrocholesterol molecule, chemically modifying it to create cholesterol.
The efficiency of the DHCR7 enzyme determines how much 7-dehydrocholesterol is converted into cholesterol versus how much remains available for other uses. This places the enzyme in a regulatory position, directing the flow of this precursor molecule toward maintaining cellular health or serving another purpose.
The Process of Vitamin D Production
While 7-dehydrocholesterol is a precursor in cholesterol synthesis, it also has a distinct role in producing vitamin D. When skin is exposed to sunlight, ultraviolet B (UVB) radiation penetrates its outer layers. The 7-dehydrocholesterol in the skin absorbs this UVB light, which sets off a photochemical reaction.
The energy from UVB photons breaks a chemical bond within the 7-dehydrocholesterol molecule, causing its structure to open and transform into pre-vitamin D3. This conversion depends on the amount of UVB radiation reaching the 7-dehydrocholesterol stored within the skin.
This newly formed pre-vitamin D3 is unstable. Under the influence of the body’s natural heat, it undergoes a heat-dependent process that rearranges its atoms into the more stable configuration known as vitamin D3 (cholecalciferol).
Once formed, vitamin D3 enters the bloodstream to be converted in the liver and kidneys into the active form the body can use. A natural safeguard prevents a vitamin D overdose from sun exposure. Both pre-vitamin D3 and vitamin D3 are light-sensitive and will break down into inactive molecules if overexposed, preventing a toxic buildup.
When Dehydrocholesterol Metabolism Fails
When the DHCR7 enzyme does not work correctly due to a genetic issue, it leads to a rare condition known as Smith-Lemli-Opitz syndrome (SLOS). SLOS is an inherited disorder caused by mutations in the DHCR7 gene, which contains the instructions for making the enzyme. An individual must inherit a mutated copy of this gene from both parents to be affected.
In individuals with SLOS, the reduced or absent activity of the DHCR7 enzyme blocks the final step of cholesterol production. This failure has two major biochemical consequences. First, the body is unable to produce enough cholesterol for normal development. Second, the precursor, 7-dehydrocholesterol, accumulates in the blood and other tissues to toxic levels.
This combination of low cholesterol and high 7-dehydrocholesterol levels disrupts growth and development, leading to the wide range of symptoms seen in SLOS. The severity of the condition often correlates with the degree of enzyme deficiency. Symptoms can vary from mild to severe but often include distinctive facial features, growth restriction, and intellectual disability. Treatment may involve cholesterol supplementation.