Urinary 3-hydroxyproline excretion in Alport's syndrome: a non-invasive screening test?

Lens capsule as a model to study type IV collagen

The study of collagen IV has benefited greatly from the seminal work conducted by Arthur Veis and colleagues over three decades ago. Through a series of electron microscopy studies focused on lens basement membrane, an appreciation was gained for the distinct network-forming properties of collagen IV. Veis correctly suggested that network assembly is a phenomenon of the non-collagenous termini of the molecule. This review seeks to document how the field advanced following these seminal conclusions, including recent discoveries regarding the molecular reinforcement of networks that support Veis' conclusions.

Glomerular basement membrane. Identification of a novel disulfide-cross-linked network of alpha3, alpha4, and alpha5 chains of type IV collagen and its implications for the pathogenesis of Alport syndrome

Glomerular basement membrane (GBM) plays a crucial function in the ultrafiltration of blood plasma by the kidney. This function is impaired in Alport syndrome, a hereditary disorder that is caused by mutations in the gene encoding type IV collagen, but it is not known how the mutations lead to a defective GBM. In the present study, the supramolecular organization of type IV collagen of GBM was investigated. This was accomplished by using pseudolysin (EC 3.4.24.26) digestion to excise truncated triple-helical protomers for structural studies. Two distinct sets of truncated protomers were solubilized, one at 4 degrees C and the other at 25 degrees C, and their chain composition was determined by use of monoclonal antibodies. The 4 degrees C protomers comprise the alpha1(IV) and alpha2(IV) chains, whereas the 25 degrees C protomers comprised mainly alpha3(IV), alpha4(IV), and alpha5(IV) chains along with some alpha1(IV) and alpha2(IV) chains. The structure of the 25 degrees C protomers was examined by electron microscopy and was found to be characterized by a network containing loops and supercoiled triple helices, which are stabilized by disulfide cross-links between alpha3(IV), alpha4(IV), and alpha5(IV) chains. These results establish a conceptual framework to explain several features of the GBM abnormalities of Alport syndrome. In particular, the alpha3(IV). alpha4(IV).alpha5(IV) network, involving a covalent linkage between these chains, suggests a molecular basis for the conundrum in which mutations in the gene encoding the alpha5(IV) chain cause defective assembly of not only alpha5(IV) chain but also the alpha3(IV) and alpha4(IV) chains in the GBM of patients with Alport syndrome.

Hydroxyl radical generation in oxygen-treated infants

OBJECTIVE

Because the hydroxyl radical is capable of oxidizing phenylalanine to O-tyrosine, we sought to determine whether increased levels of O-tyrosine are found in urine of infants treated with supplemental oxygen.

METHODS

A total of 39 consecutively admitted neonates to an intensive care unit were included. Twenty-seven received supplemental oxygen therapy for respiratory disease, and 12 did not. Urinary O-tyrosine levels were determined on two or more occasions using high-performance liquid chromatography with results expressed as a percentage of the urinary phenylalanine concentration. Using simple and stepwise multiple linear regression analyses, urinary O-tyrosine was examined for associations with relevant clinical conditions and laboratory measurements.

RESULTS

Infants supplemented with oxygen showed significantly higher mean +/- SEM urinary O-tyrosine levels (0.40% +/- 0.028) compared with those remaining in room air (0.18% +/- 0.012). Mean daily FIO2 was the clinical and laboratory variable most highly correlated with urinary O-tyrosine (r = 0.66). In the stepwise regression, significant associations were also found for renal fractional sodium excretion and Apgar score at 5 minutes.

CONCLUSIONS

Hydroxylation at the O position of phenylalanine, a specific direct marker for the hydroxyl radical attack, was strongly associated with oxygen treatment in neonates. This finding increases our understanding of the pathogenesis of oxygen injury and suggests a basis for developing therapeutic approaches.

Aromatic hydroxylation in nasal lavage fluid following ambient ozone exposure

Ozone at ambient concentrations affects lung function and initiates an inflammatory response of the airways. However, the underlying mechanisms are poorly understood. In vitro studies have shown that ozone reacts with water to give reactive hydroxyl radicals capable of oxidizing a wide range of biomolecules. We conducted a study to determine if in vivo hydroxyl radical attack on human airways occurs under natural exposure to ozone. The relation of orthotyrosine to para-tyrosine as a measure of hydroxyl radical attack was analyzed in nasal lavage samples of 44 primary school children in an epidemiologic study. Repeated nasal lavages were performed between May and October 1991 both following "low" (daily half-hour maximum < 140 micrograms/m3, approximately 70 ppb) and "high" (daily half-hour maximum > 180 micrograms/m3, approximately 90 ppb) ozone exposure. Concomitantly, lung function tests were performed. On average, 11.6 (6-16) nasal lavages were performed for each of 24 study days (10 days following "low" ozone exposure and 14 days following "high" ozone exposure). Average ortho-tyrosine (median; 5-95% percentile) for each child was 0.037 mumol/L (0.016-0.064 mumol/L) and average para-tyrosine was 15.7 mumol/L (9.8-24.1 mumol/L). Ortho-tyrosine (as percentage of tyrosine) was significantly higher following days with "high" ozone exposure (0.18%) vs. days following "low" ozone exposure (0.02%; p = .0001). Ortho-tyrosine showed an inverse relationship with forced vital capacity (p = .01) but was not related to inflammation of the upper airways as assessed by cell counts of polymorphonuclear neutrophils. Hydroxyl radical attack subsequent to ambient ozone occurs in the upper airways of healthy children and is related to lung function decrements.

Evidence for McKusick's hypothesis of deficient collagen cross-linking in patients with homocystinuria

Osteoporosis occurs commonly in homocystinuria. The underlying pathobiochemical mechanism remains unclear; disturbed cross-linking of collagen has been suggested but this hypothesis has not been fully tested, nor have studies on collagen synthesis been performed. We therefore used recently available noninvasive tests for collagen synthesis and cross-linking to examine 10 patients with homocystinuria. Synthesis of collagen type I and type III was not different from age-matched healthy controls as reflected by comparable plasma levels of carboxyterminal propeptide of type I procollagen (PICP) and of plasma levels of N-terminal propeptide of procollagen type III (PIIINP). Collagen type I cross-links expressed by serum carboxyterminal telopeptide of collagen type I (ICTP) were 1.14 +/- 0.24 micrograms/l in the patient group versus 3.29 +/- 0.32 micrograms/l in the control group. This significant reduction of cross-links in the group with homocystinuria did not correlate with serum homocysteine or homocysteic acid concentrations. Our data clearly indicate that the disturbed cross-linking hypothesis still holds and that the bone manifestations of homocystinuria are not due to deficient collagen synthesis.

Alpha-methyl-proline restores normal levels of bone collagen Type I synthesis in ovariectomized rats

Collagen type I is the major protein of bone matrix and significantly reduced in osteoporosis. We tested the effect of alpha - methyl - proline on collagen synthesis in the model of the ovariectomized rat. Collagen synthesis was studied at the transcriptional level using Northern and dot blotting and at the protein level using hydroxyproline determination and a specific dye binding collagen assay. Alpha - methyl- proline treatment significantly increased collagen synthesis as compared to untreated ovariectomized and estradiol treated ovariectomized rats and restored collagen synthesis to levels of sham operated rats. Proline analogues were described to stimulate procollagen synthesis at the transcriptional level, however, if incorporated, lead to negative collagen production due to rapid intracellular degradation of the deficient collagen. Our synthesized analogue is not being incorporated, thus not interfering with collagen conformation and can therefore induce collagen production.