As a major part of collagen, hydroxyproline has an essential role in collagen stability. Small amounts of hydroxyproline are also found in the following:
- The C1q component of the complement system
- The macrophage scavenger proteins
- Ectodysplasin A
Two isomeric forms of hydroxyproline have been identified in collagen. Trans-4-hydroxy-L-proline is found in type I and type III collagen, whereas higher amounts of trans-3-hydroxy-L-proline are found in type IV collagen. Normal secretion of procollagen molecules out of the cells requires a critical amount of trans-4-hydroxy-L-proline. Therefore, without hydroxyproline, no functional collagen fibers appear in the extracellular space.
Hydroxyproline can be tested in the serum and in the urine. The serum level of hydroxyproline excretion correlates well with its urinary level. However, urinary hydroxyproline better discriminates (x 100) between normal and metastatic disease of the bone.
In addition to the substrate, hydroxylation of peptidyl prolyl and peptidyl lysyl residues, catalyzed by prolyl hydroxylase and lysyl hydroxylase of skin, skeletal muscle, and granulating wounds, requires molecular oxygen, ascorbate, Fe2+, and alpha-ketoglutarate. A vitamin C deficiency can lead to scurvy, which produces bleeding gums, swelling joints, and impaired wound healing result due to the impaired collagen stability.
Hydroxyproline testing is indicated for the following:
- Paget disease of bone or to detect the inflammatory or neoplastic involvement of the bone
- As an indicator of activity in acromegaly and response to treatment to growth hormone
- As a diagnostic aid in Marfan syndrome
Hyperhydroxyprolinemia is a benign condition caused by a defect in 4-hydroxyproline dehydrogenase. No impairment of proline catabolism is associated with this condition.
The ratio of hydroxyproline/creatinine in a fresh specimen of early morning urine, considered the total urinary hydroxyproline, is the best index of collagen breakdown in metastatic bone disease. This measurement is preferable to free serum hydroxyproline measurement in this setting.
Although serum hydroxyproline and urinary hydroxyproline measurements are strongly correlated, urinary hydroxyproline measurement in 24-hour or spot urine is easier to collect. Spot urine testing is measurement of choice in screening for bony involvement by malignancy because collagen intake does not influence hydroxyproline/creatinine.
In individuals with bony metastases (most notably those with prostate cancer), increases in serum and urine hydroxyproline levels are observed prior to radiographic evidence of metastatic lesions; thus, these measurements are potential markers of skeletal metastatic disease and may be recommended in settings where advanced imaging modalities are not readily available. Moreover, hydroxyproline level measurements may permit illness monitoring and assessment of treatment response in such settings.
Hydroxyproline is a collagen related amino acid. Hydroxyproline is a nonessential amino acid, which means that it is manufactured from other amino acids in the liver. Hydroxyproline is necessary for the construction of the body’s major structural protein, collagen. Hydroxyproline is present in essentially all tissues and all genetic types of collagen. 
Defects in collagen synthesis lead to easy bruising, internal bleeding, breakdown of connective tissue of the ligaments and tendons, and increased risk to blood vessel damage.
Hydroxyproline’s immediate precursor is proline. The conversion of proline to hydroxyproline requires vitamin C (ascorbic acid). Vitamin C deficiency results in the poor conversion of proline to hydroxyproline.
Serge Cremers, Patrick Garnero, Markus J Seibel, Chapter 87 - Biochemical Markers of Bone Metabolism, Editor(s): John P. Bilezikian, Lawrence G. Raisz, T. John Martin, Principles of Bone Biology (Third Edition), Academic Press, 2008, Pages 1857-1881 
Wheeless' Textbook of Orthopaedics Presented by Duke Orthopaedics, Pagets Disease 
Laitinen O, Nikkila EA, Kivirikko KI. Hydroxyproline in the serum and urine. Normal values and clinical significance. Acta medica Scandinavica. Mar 1966. 179(3):275-284. 
Klein L. Urinary Hydroxyproline during Late Pregnancy and Postpartum Involution. Metabolism: clinical and experimental. May 1964. 13:386-390. 
Krane SM, Simon LS. Metabolic consequences of bone turnover in Paget's disease of bone. Clin Orthop Relat Res. Apr 1987. (217):26-36. 
Wheeless, CM. Wheeless Textbook of Orthopaedics. United States: 1996. 386-390. 
Pagets Disease. Wheeless, CR, Urbaniak JR, eds. Wheeless' Textbook of Orthopaedics. 1996. 
Gasser AB, Depierre D, Courvoisier B. Total urinary and free serum hydroxyproline in metastatic bone disease. Br J Cancer. Mar 1979. 39(3):280-283. 
- Collagen synthesis requires iron, alpha-ketoglutarate and vitamin C. Chondroitin sulfate and manganese can also be helpful.
- Elevated urinary hydroxyproline is a reflection of bone lysis and may also occur in RA, hyperparathyroidism, and OM; 
- Increased serum and urine levels of hydroxyproline have been demonstrated in Paget’s disease 
- Paget disease
- Rheumatoid arthritis
- Polyarteritis nodosa
- Marfan syndrome
- Turner syndrome
- Skeletal metastases
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