Hydroxyproline
Updated: May 05, 2022
Author: Bishnu Prasad Devkota, MD, MHI, FRCS(Edin), FRCS(Glasg), FACP, FAMIA; Chief Editor: Sridevi Devaraj, PhD, DABCC, FAACC, FRSC, CCRP
As a major part of collagen, hydroxyproline has an essential role in collagen stability. Testing hydroxyproline in the serum and in the urine is common.
The reference range of hydroxyproline is as follows:[1]
Total hydroxyproline in the urine among those aged 18-21 years - 13-28 mg/24/m2
Total hydroxyproline in the urine among those aged 22-55 years - 8.5-23.5 mg/24/m2
Free hydroxyproline in the serum of males - 0.7-1.55 µg/mL
Free hydroxyproline in the serum of females - 0.7-1.40 µg/mL
The following conditions are associated with increased hydroxyproline levels:[1, 2, 3, 4, 5, 6, 7]
Paget disease
Rheumatoid arthritis
Hyperparathyroidism
Osteomyelitis
Polyarteritis nodosa
Marfan syndrome
Acromegaly
Hyperthyroidism
Turner syndrome
Skeletal metastases
Pregnancy
Graft versus host disease
Vitiligo
Keloids
Specimen: Blood, urine
Container: Blue-top vacuum tube
A sealed, leak-proof container marked with a biohazard sticker is required to comply with Occupational Safety and Health Administration (OSHA) safety standards.
As a major part of collagen, hydroxyproline has an essential role in collagen stability. Small amounts of hydroxyproline are also found in the following:[8]
Elastin
Acetylcholinesterase
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.[8] 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.[8]
A study of vertebrate and invertebrate species by Taga et al indicated that 4-hydroxyproline in the Yaa position of collagen peptides is the dominant contributor to collagen’s thermal stability, particularly in vertebrates. The investigators concluded this after finding that 4-hydroxyproline in the Yaa position has better positive correlation with collagen’s denaturation temperature than does the total amount of hydroxyproline (including 3- and 4-hydroxyproline) in the Xaa position.[9]
Hydroxyproline can be tested in the serum and in the urine. The serum level of hydroxyproline excretion correlates well with its urinary level.[1] However, urinary hydroxyproline better discriminates (x 100) between normal and metastatic disease of the bone.[6]
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.[8]
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[1]
As a diagnostic aid in Marfan syndrome[1]
Hyperhydroxyprolinemia is a benign condition caused by a defect in 4-hydroxyproline dehydrogenase. No impairment of proline catabolism is associated with this condition.[10]
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.[6]
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.[11] Moreover, hydroxyproline level measurements may permit illness monitoring and assessment of treatment response in such settings.[11]