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Scurvy: Biochemistry and Diagnosis

It's commonly known that scurvy is caused by vitamin C deficiency. The Latin name for the disease is scorbutus, and the name "ascorbic acid" (for vit C) is derived from this name of the disease. It was first described by Hippocrates about 300-400 BC, and was common among pirates and naval sailors until the 1900's.

Scurvy is sometimes called Barlow's disease when present in infants, and has a few other names. It is rare among adults in modern 1st world nations, but more common at a sub-acute level among infants and the elderly, however it is possible to cause scurvy by eating a narrow high protein diet and avoiding fruit. Infants who are fed pasteurized bottled milk can be deficient because vitamin C is destroyed by heat (pasteurization). Scurvy is a component of malnutrition.

In addition to being bounteous in citrus, vitamin C is present in many fruits and vegetables including guava, papaya, tomatoes, onions, strawberries, bell peppers, broccoli, potatoes, cabbage, spinach, paprika, and pickles. Vitamin C is an antioxidant, meaning that it can prevent or relieve oxidative stess in the body. Oxidative stress is a contributor to many diseases including Alzheimer's, Parkinson's and atherosclerosis.

British sailors became known as Limeys because they carried lemons and limes on their seagoing vessels so the crew could avoid scurvy. Citrus keeps well in its own thick skin and completely prevents the disease. Sauer kraut also works as a preventative, and the German navy used it, getting named "Krauts" because of it. It was not known that vitamin C was the essential nutrient to prevent scurvy until 1932.

Scurvy is characterized by pale skin, scorbutic or spongy gums, liver spots and petechiae on the skin, bleeding from mucous membranes, and sometimes depression and joint pain. Petechiae are tiny red spots resulting from blood leaking from the capillaries into the skin, usually around hair follicles. In advanced stages the skin develops open suppurating lesions and the teeth fall out. Here's a typical liver spot pattern on some hairy legs:

The biochemical reason for scurvy is that without vitamin C, we cannot convert procollagen to collagen. Collagen is normally synthesized at the endoplasmic reticulum of a cell, where proline and lysine residues must by hydroxylated. The enzymes that catalyze the hydroxylation require ascorbic acid as a co-factor to perform their function. (Remember, proline and lysine are amino acids, components of protein.)
and this is lysine:

Collagen makes up about 25% of the protein in mammals, has provides much of the strength in bones, teeth, fascia, cartilage, ligaments and tendons. It has more tensile strength (per weight) than steel. Collagen gives elasticity to bone, making it much harder to break. Collagen fibers are part of the extracellular matrix that structurally supports most tissues and cells. Collagen is also found inside certain cells, such as the cornea and lens of the eye, and the skin. The degradation of collagen (and keratin) causes the wrinkles of aging. It is used in cosmetic and burn surgery.

When infants can't manufacture enough of it their bones are weak and may bow out like this:

Tropocollagen, or a collagen molecule, is a coiled coil that is covalently cross-linked. The crosslinking increases with age, which is why meat from older animals is tougher than meat from younger animals. Lamb is my favorite. Tropocollagen is comprised of three lefthanded coils, wrapped together in a right handed fashion. Each of the chains has a distinctive pattern of amino acids, usually glycine-x-proline or glycine-x-hydroxyproline. The x can be any amino acid, because it sticks out on the outside. Glycine is the smallest aa and occurs on the inside of the coiled coil. No other aa, not even alanine, will fit. Glycine has just a hydrogen for its side chain:

Proline (several paragraphs above) and hydroxyproline (same with an -OH added) are non-reactive aa's because of the inflexible ring structure, and that is fine for a structural protein. A globular protein such as an enzyme would not have a repeating pattern like this, nor the high proportion of non-reactive aa's.

Silk fibroin is the only other fibrous protein that has similar regular repetitions and high glycine content. Silk is 75-80% glycine alternating with alanine, and about 10% serine.

In bone, collagen triple helices are arranged differently leaving gaps where long, hard, fine mineral crystals deposit. The mineral is something like Ca5(PO4)3(OH) and its deposition causes certain kinds of cartilage to turn into bone. I know from my A&P class last semester that the hormone estrogen speeds up bone maturation, and I wonder if it is this process that it helps along.

--vit C deficiency depresses formation of CT, cartilage and bone, causes capillary fragility
--4-10 months to develop
--few radiographic signs in adult, osteopenia mainly
--latent for months, affects children most age 4-14 months
--S/Sx; spontaneous hemorrhage, swelling, irritability, pain, lying motionless (frog legged), costal rosary (scorbutic rosary), serum ascorbic acid 0.6mg/100ml
--X-ray: abnormalities at growing ends of long bones are most characteristic
--dense zone of provisional calcification (white line of frankel) is sclerotic edge of metaphysis
--ring epiphysis (Wimberger's sign) distal femoral epiphysis is centrally demineralized with a peripheral mineralized ring

--scorbutic zone (Trummerfeld zone) Metaphyses show a characteristic transverse lucent band (Trummerfeld's zone) representing the primary spongiosa and relative increased density of the adjacent zone of provisional calcification (white line of Frankel).

--subperiosteal hemorrhages (dt vascular fragility)



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