Effects of dietary hydroxyproline on collagen metabolism, proline 4-hydroxylase activity, and expression of related gene in swim bladder of juvenile Nibea diacanthus

This study was conducted to investigate the effects of dietary hydroxyproline (Hyp) on tissue collagen level, proline 4-hydroxylase (P4H) activity as well as transcript levels of COL1As (COL1A1 and COL1A2) and P4Hαs (P4Hα(I), P4Hα(II), and P4Hα(III)) in juvenile Nibea diacanthus. A total of 450 fishes were randomized to six equal groups and fed the diet with graded supplementary Hyp-0, 5, 10, 15, 20, and 25 g kg^-1 of dry matter for 8 weeks. Results showed that fish fed diets with 10 g kg^-1 Hyp had significantly higher acid-soluble collagen (ASC) and total collagen (TC) concentrations in swim bladder than fish fed with the other diets (P < 0.05). The activity of P4H in liver and swim bladder showed a similar trend, showing first increase and then decrease with increasing dietary Hyp (P < 0.05). The mRNA expression of COL1As in swim bladder and muscle were significantly higher than those in the liver and intestines. Meanwhile, with increasing dietary Hyp, the relative expression of COL1As genes in swim bladder showed a similar pattern with the TC concentrations of swim bladder, increased significantly initially followed by a decrease. Increased dietary Hyp content corresponded with significant decrease in the mRNA level of P4Hαs in swim bladder. These results indicated that the dietary Hyp promotes the collagen accumulation of swim bladder to some extent, and the promoting action may be related to the expression of COL1As. The optimum supplement of dietary Hyp was estimated from TC of swim bladder with piecewise regression analysis to be 9.66 g kg^-1.

Improved methodology to induce hyperoxaluria without treatment using hydroxyproline

The use of hydroxyproline (HP) to generate hyperoxaluria in the rat is a problem because it is impossible to separate the effect of oxalate on renal injury from the effects of HP and the large array of metabolic intermediates formed when HP is converted to oxalate. Previously, the Dahl salt-sensitive (SS) and Brown Norway (BN) rat strains were studied to determine genetic control of resistance or susceptibility to HP-induced renal injury and crystal deposition. To develop a better model to induce hyperoxaluria without causing injury from HP metabolites, animals were fed a diet containing various levels of added oxalate (0, 1, 2, 3, or 5%). After 5 weeks rats were killed and the kidneys were removed for microscopic evaluation of tubule changes and crystal deposition. The 3 and 5% oxalate-fed groups had a substantial increase in urine oxalate, about 50 and 140 μmol/g body weight over controls, respectively. Both the SS and BN 3% oxalate-fed animals showed only slightly elevated tubule area and no crystal deposition. However, BN animals fed 5% oxalate had a dramatic increase in their percent tubule areas compared to control BN rats and treated SS rats. Crystal deposition in the kidneys was only observed in the 5% oxalate-fed groups. The BN kidneys demonstrated a threefold higher crystal deposition compared to oxalate-fed SS rats. We conclude that oxalate-supplemented food is a better method of producing hyperoxaluria in the rat than using HP which may introduce metabolic intermediates injurious to the kidney.

Experimental induction of calcium oxalate nephrolithiasis in mice

PURPOSE

The availability of various transgenic and knockout mice provides an excellent opportunity to better understand the pathophysiology of calcium oxalate stone disease. However, attempts to produce calcium oxalate nephrolithiasis in mice have not been successful. We hypothesized that calcium oxalate nephrolithiasis in mice requires increasing urine calcium and oxalate excretion, and experimentally induced hyperoxaluria alone is not sufficient. To provide evidence we induced hyperoxaluria by administering hyperoxaluria inducing agents in normocalciuric and hypercalciuric mice, and investigating various aspects of nephrolithiasis.

MATERIALS AND METHODS

We administered ethylene glycol, glyoxylate or hydroxyl proline via diet in male and female normocalciuric B6 mice, and in hypercalciuric sodium phosphate co-transporter type 2 a -/- mice for 4 weeks. We collected 24-hour urine samples on days 0, 3, 7, 14, 21 and 28, and analyzed them for pH, creatinine, lactate dehydrogenase calcium and oxalate. Kidneys were examined using light microscopy. Urine was examined for crystals using light and scanning electron microscopy.

RESULTS

Hypercalciuric mice on hydroxyl proline did not tolerate treatment and were sacrificed before 28 days. All mice on ethylene glycol, glyoxylate or hydroxyl proline became hyperoxaluric and showed calcium oxalate crystalluria. No female, normocalciuric or hypercalciuric mice showed renal calcium oxalate crystal deposits. Calcium oxalate nephrolithiasis developed in all mice on glyoxylate and in some on ethylene glycol. In all mice the kidneys showed epithelial injury. Male mice particularly on glyoxylate had more renal injury and inflammatory cell migration into the interstitium around the crystal deposits.

CONCLUSIONS

Results confirm that hyperoxaluria induction alone is not sufficient to create calcium oxalate nephrolithiasis in mice. Hypercalciuria is also required. Kidneys in male mice are more prone to injury than those in female mice and are susceptible to calcium oxalate crystal deposition. Perhaps epithelial injury promotes crystal retention. Thus, calcium oxalate nephrolithiasis in mice is gender dependent, and requires hypercalciuria and hyperoxaluria.

Vitamin B6 deficiency augments endogenous oxalogenesis after intravenous l-hydroxyproline loading in rats

The effects of an intravenous hydroxyproline load on endogenous oxalogenesis were compared in rats fed a standard diet or a vitamin B6-deficient diet. Twelve male Wistar rats were randomized to two groups and were fed either a standard diet (control group) or a vitamin B6-deficient diet for 3 weeks. Then the animals were intravenously administered 100 mg (762.6 micromol)/ml hydroxyproline. In the control group, infusion of hydroxyproline increased the 5-h urinary oxalate and glycolate excretion above baseline to 0.27% (2.02 +/- 1.11 micromol) and 0.32% (2.43 +/- 1.60 micromol) of the administered dose (mol/mol), while it was respectively 2.01% (15.24 +/- 2.13 micromol) and 0.00% (-0.02 +/- 0.19 micromol) of the dose in the vitamin B6-deficient group. Therefore, vitamin B6 deficiency augmented endogenous synthesis of oxalate from hydroxyproline by 7.56-fold (15.24/2.02) compared with that in the control group. Urinary citrate excretion was significantly lower at baseline and all other times in the vitamin B6-deficient group compared with the control group. In conclusions, L-hydroxyproline loading augmented endogenous oxalogenesis in the vitamin B6-deficient group without causing hyperglycolic aciduria, and also led to significant hypocitraturia. These findings suggest that hydroxyproline is not metabolized to oxalate via glycolate, but rather via the 4-hydroxyglutamate to glyoxylate pathway (usually requiring vitamin B6-dependent enzymes) even in the presence of vitamin B6 deficiency.

Prodrug of proline analogue reduces hypoxic pulmonary hypertension in rats

A polymeric prodrug of the proline analogue cis-4-hydroxy-l-proline (CHOP), poly(ethylene glycol)-lysine-CHOP or CHOP-PEG, prevents hypoxic pulmonary hypertension in rats by inhibiting collagen accumulation. A more potent prodrug was synthesized by increasing the loading of CHOP on the carrier from 14 to 100%. Pulmonary antihypertensive efficacy and pharmacokinetics are described in the rat hypoxia model. The antihypertensive effect of CHOP-PEG in rats exposed to 10% O2 for 7d showed approximately 2 x 10(2)-fold greater potency than monomeric CHOP. Routes of administration were compared to determine the lowest dose of CHOP-PEG that reduced right ventricular pressure approximately 50% vs. untreated hypoxic controls at 7d. Total doses required were: continuous s.c. via an osmotic minipump, 0.8 mg; single s.c., 10mg; single i.v., 40 mg; and single intratracheal 90 mg. Efficacy for at least 7d postdosing in pre-established pulmonary hypertension was shown. Using an ELISA-based assay, biphasic i.v. and stable s.c. pharmacokinetic profiles were observed 72 h after single injections and 7d after continuous s.c. infusion. Thus, this CHOP-PEG formulation prevents and reverses chronic hypoxic pulmonary hypertension in rats, is most effective when given by continuous s.c. infusion, and has favorable pharmacokinetic properties. Potent inhibitors of fibrosis appear to be promising agents in treating pulmonary hypertension and possibly other fibrosing diseases.

Sustained release of cis-hydroxyproline in the treatment of experimental proliferative vitreoretinopathy in rabbits

PURPOSE

To evaluate the efficacy of sustained-release cis-4-hydroxyproline (CHP), a proline analog that inhibits collagen secretion, on experimental proliferative vitreoretinopathy (PVR) in rabbits.

METHODS

To demonstrate the sustained release of CHP we developed scleral implants weighing 8.5 mg made of a homogeneous mixture of poly( D, L-lactide-co-glycolide) (PLGA) and various doses of CHP. The CHP release profiles were evaluated by high-performance liquid chromatography in vitro. Scleral implants loaded with 20% and 15% of CHP and made from PLGA (copolymer ratios 65/35 and 50/50; mean molecular weights 103,000 and 93,000, respectively) were used to treat experimental PVR and the efficacy was studied. In treated eyes, two PLGA 65/35 implants ( n=7), PLGA 50/50 implants ( n=6), or a PLGA 65/35 and a PLGA 50/50 implant ( n=9) were inserted at the pars plana, followed by PVR induction with autologous fibroblasts. Control eyes ( n=18) received two implants without CHP. Ocular tissue toxicity was evaluated histologically.

RESULTS

In vitro release studies demonstrated a triphasic release pattern. The PLGA 65/35 and PLGA 50/50 implants released CHP over 4 and 7 weeks, respectively. The PLGA 65/35 implants decreased the incidence of retinal detachment from 89% (in controls) to 57% on day 28. When both PLGA 65/35 and PLGA 50/50 implants were used, the inhibitory effect was synergistically enhanced ( p=0.0034), while implantation with only PLGA 50/50 implants had no significant effect on PVR. No toxic reactions were observed.

CONCLUSION

These results suggest that the biodegradable polymeric implants containing CHP represent a potential treatment for PVR.

Calcium oxalate stone formation in genetic hypercalciuric stone-forming rats

BACKGROUND

Over 54 generations, we have successfully bred a strain of rats that maximizes urinary calcium excretion. The rats now consistently excrete 8 to 10 times as much calcium as controls, uniformly form poorly crystalline calcium phosphate kidney stones, and are termed genetic hypercalciuric stone-forming (GHS) rats. These rats were used to test the hypothesis that increasing urinary oxalate excretion would not only increase the supersaturation with respect to the calcium oxalate solid phase, but also would increase the ratio of calcium oxalate-to-calcium phosphate supersaturation and result in calcium oxalate stone formation.

METHODS

To increase urine oxalate excretion an oxalate precursor, hydroxyproline, was added to the diet of male GHS rats. The GHS rats were fed a standard 1.2% calcium diet alone or with 1%, 3% or 5% trans-4-hydroxy-l-proline (hydroxyproline).

RESULTS

The addition of 1% hydroxyproline to the diet of GHS rats led to an increase in urinary oxalate excretion, which did not increase further with the provision of additional hydroxyproline. The addition of 1% and 3% hydroxyproline did not alter calcium excretion while the provision of 5% hydroxyproline led to a decrease in urine calcium excretion. The addition of 1% hydroxyproline led to an increase in urinary calcium oxalate supersaturation, which did not further increase with additional hydroxyproline. The addition of 1% and 3% hydroxyproline did not alter urinary supersaturation with respect to calcium hydrogen phosphate while the addition of 5% hydroxyproline tended to lower this supersaturation. Compared to rats fed the control and the 3% hydroxyproline diet the addition of 5% hydroxyproline increased the ratio of calcium oxalate supersaturation to calcium phosphate supersaturation. Virtually all rats formed stones. In the control and 1% hydroxyproline group, all of the stones were composed of calcium and phosphate (apatite), in the 3% hydroxyproline group the stones were a mixture of apatite and calcium oxalate, while in the 5% hydroxyproline group all of the stones were calcium oxalate.

CONCLUSIONS

The provision of additional dietary hydroxyproline to GHS rats increases urinary oxalate excretion, calcium oxalate supersaturation and the ratio of calcium oxalate-to-calcium phosphate supersaturation, resulting in the formation of calcium oxalate kidney stones. Thus, with the addition of a common amino acid, the GHS rats now not only model the most common metabolic abnormality found in patients with nephrolithiasis, hypercalciuria, but form the most common type of kidney stone, calcium oxalate.

cis-hydroxyproline limits work necessary to flex a digit after tendon injury

Present surgical procedures for the repair of tendon injury are complicated by formation of peritendinous collagenous adhesions which restrict tendon gliding. Several sensitive biomechanical tests have been developed to quantitate peritendinous restriction of gliding. Using these tests it was found that cis-hydroxyproline significantly limits net collagen accumulation following tendon injury in a rat model and decreases the adhesive restraints to tendon gliding. cis-Hydroxyproline and other proline analoques offer a method for specifically preventing excessive collagen formation.

Oxaceprol is as effective as diclofenac in the therapy of osteoarthritis of the knee and hip

In this multicentre (five centres in Germany), randomised, double-blind, comparative study, 150 patients with painful degenerative joint disease according to EULAR criteria received either oxaceprol (200 mg three times daily) or diclofenac (25 mg three times daily) for 20 days. Joint function, the primary variable, assessed according to Lequesne's indices, improved equally in both treatment groups to a clinically relevant degree. Joint mobility improved by approximately 60% in both groups. By the end of therapy in both groups, the period of pain-free walking time had more than doubled and subjectively evaluated pain perception (VAS) was reduced by almost 50% without any significant differences between the treatments. The incidence of adverse drug reactions was similar in both groups but oxaceprol induced milder symptoms. Oxaceprol is as effective and better tolerated than diclofenac in the treatment of osteoarthritis.

Polymer of proline analogue with sustained antifibrotic activity in lung fibrosis

Inhibitors of collagen such as cis-4-hydroxy-L-proline (cHyp) may ameliorate bleomycin (bleo)-induced pulmonary fibrosis. An alternating polymer of poly(ethylene glycol) (PEG)-lysine (PEG-Lys) with cHyp attached as a pendant side chain was prepared for intratracheal delivery with bioinactive trans-Hyp (tHyp) polymer as control. To test whether the cHyp polymer has prolonged lung retention and sustained antifibrotic activity, we first instilled 3H- and 14C-labeled cHyp polymer in normal rats. Lung retention was 86 +/- 9% at 6 h and 29 +/- 3% at 7 d (n = 5). Next, rats were instilled intratracheally with either saline (sal) or 1.2 U bleo, and the following treatment groups were studied: Bleo/sal; Bleo/cHyp polymer; Bleo/tHyp polymer; and Bleo/PEG-Lys + cHyp. The dose of the test agents was 150 mg/kg polymer containing 8.5 mg/kg cHyp or tHyp instilled intratracheally at 7 and 14 d after bleo. At 21 d, hydroxyproline content (mg/lung) was: Control, 1.8 +/- 0.1; Bleo/sal 4.0 +/- 0.1*; Bleo/cHyp polymer, 2.8 +/- 0.3*+; Bleo/tHyp polymer, 4.4 +/- 0.2*; and Bleo/PEG-Lys + cHyp, 4.0 +/- 0.1* (*p < 0.05 versus Control; +p < 0.05 versus Bleo/sal; n = 5/group). The cHyp polymer also reduced lung total protein content, but the decrease was not significant. The dose required to produce 50% inhibition of lung collagen was approximately 700-fold less than monomeric cHyp. Thus, the cHyp polymer is a potent, long-acting antifibrotic agent which may be useful in treating lung fibrosis.

Polymeric carrier of proline analogue with antifibrotic effect in pulmonary vascular remodeling

The proline analogue cis-4-hydroxy-L-proline (cHyp) inhibits collagen accumulation but diffuses out of tissues. To prolong the antifibrotic effect, we used a copolymer of cHyp attached to a backbone of poly(ethylene glycol) (PEG) and lysine. The copolymer was encapsulated in liposomes conjugated with PEG or in liposomes coated with the polysaccharide amylopectin to improve uptake by lungs after intravenous infusion. Amylopectin-liposomes had approximately 3-fold greater uptake in cultured endothelial cells compared with PEG-liposomes and greater lung retention 1 wk after infusion (5.2 +/- 0.8% versus 2.7 +/- 0.2%, p < 0.05). Sustained antifibrotic activity, assayed by growth inhibition of smooth muscle cells and fibroblasts over 4 d, was greater for amylopectin-liposomes/copolymer than PEG-liposomes/copolymer. Inhibition of collagen accumulation in pulmonary arteries of hypoxic (10% O2) rats was used to assess antifibrotic activity. Amylopectin-liposomes/copolymer attenuated increased right ventricular pressure by approximately 50% and completely prevented excess vascular collagen 1 wk after a single intravenous injection. The copolymer in liposomes was > 1,000-fold more effective by weight than unencapsulated monomeric cHyp. Thus, the copolymer, a potent, long-acting antifibrotic agent, totally prevented collagen accumulation for 1 wk in pulmonary arteries undergoing vascular remodeling when delivered in amylopectin-liposomes.

Liposome encapsulation improves the effect of antifibrotic agent in rat lung fibrosis

We studied whether the therapeutic efficacy of the antifibrotic agent cis-4-hydroxy-L-proline (cHyp) in preventing bleomycin-induced pulmonary fibrosis in rats is enhanced by intratracheal delivery in liposomes. Dual-radiolabeled liposomes were used to study the distribution and stability of liposomes after intratracheal instillation. Lung retention was > 20% 1 wk after intratracheal instillation of 9 mumol phospholipid, and liposomes were intact as indicated by the ratio of the lipid and aqueous-phase markers remaining unchanged. For the fibrosis study, groups of rats were instilled with 1.2 U bleomycin (Bleo) and treated 1 and 2 wk later by single intratracheal instillation of test compounds. The control group received 0.3 ml saline (Bleo/sal). The treated groups received 9 mumol phospholipid in 0.3 ml of the following liposome preparations: empty liposomes (Bleo/lip), liposomes and 100 mg/kg of free unencapsulated cHyp (Bleo/lip/cHyp), and 100 mg/kg of liposome-encapsulated cHyp (Bleo/lip-cHyp). At 3 wk, fibrosis (mg hydroxyproline/g weight lung) by groups was as follows: control, 2.6 +/- 0.1 (SEM); Bleo/sal, 3.2 +/- 0.1, Bleo/lip, 3.2 +/- 0.1, and Bleo/lip/cHyp, 3.1 +/- 0.1, p < 0.05 compared with control; Bleo/lip-cHyp, 2.6 +/- 0.1, p < 0.05 compared with Bleo/sal, n = 3 to 6. Histologic grading of fibrosis did not show decreased fibrosis in the Bleo/lip-cHyp group, probably because of the focal nature of the fibrotic lesions. We conclude that cHyp encapsulated in liposomes prevents bleomycin-induced fibrosis by biochemical measurements. Delivery of antifibrotic agents to the lung in carrier vehicles promotes retention and may enhance their efficacy in treating bleomycin-induced pulmonary fibrosis.

Liposome-entrapped antifibrotic agent prevents collagen accumulation in hypertensive pulmonary arteries of rats

We studied the therapeutic efficacy of an intravenously injected antifibrotic agent encapsulated in liposomes on inhibiting collagen accumulation in hypertensive blood vessels. cis-4-Hydroxy-L-proline (cHyp) in liposomes was injected into rats exposed to 10% O2, and drug effect was evaluated by measuring right ventricular pressure and hydroxyproline content of the pulmonary artery. Right ventricular pressure was 11 +/- 1 mm Hg (mean +/- SEM) 5 days after a single intravenous injection of 200 mg/kg cHyp in liposomes compared with 14 +/- 1 mm Hg in rats injected with empty liposomes; hydroxyproline content was also reduced by cHyp treatment (87 +/- 6 versus 107 +/- 7 micrograms per vessel) (p less than 0.05 for both, n = 6-9). Injections of cHyp in liposomes every 5 days partially prevented hypertension and vascular collagen accumulation during a 3-week exposure to hypoxia, and the dose required was one tenth the dose of unencapsulated cHyp. Therapeutic doses of cHyp in liposomes injected for 6 months affected tensile properties of main pulmonary artery and aorta, but there were no apparent histological effects on other organs. Liposomes injected intravenously were identified in pulmonary artery endothelial cells. The prolonged effect of a single injection of cHyp in liposomes may be due to uptake of the liposomes by the endothelium. Liposome delivery of drugs to the arterial wall may be useful in the study and treatment of hypertensive vascular disease.

Synthesis and characterization of magnetically responsive albumin microspheres containing cis-hydroxyproline for scar inhibition

Magnetically responsive albumin microspheres containing the proline analog, cis-hydroxyproline, were synthesized and their in-vitro physical properties were characterized. These microspheres have an average size of 1.1 +/- 0.3 microns with 94% of the microspheres less than 2.0 microns. They are uniformly spherical and contain 67% albumin, 22% magnetite, and 8% cis-hydroxyproline. The cis-hydroxyproline is releasable during a 72-h period and demonstrates a slow, releasable pool that constitutes approximately 50% of the incorporated drug. cis-Hydroxyproline is neither chemically nor biologically altered during its incorporation into the microsphere. cis-Hydroxyproline release from microspheres results in the in vitro inhibition of collagen secretion, which is indistinguishable from unincorporated cis-hydroxyproline. In a rat-tail animal model these microspheres were selectively targeted using an external magnetic field applied to a 1.5-2.0-cm target site. Fifty to 80% of the infused microspheres were localized to this site, whereas without a magnetic field only 15-20% of the microspheres are localized to this site. cis-Hydroxyproline microspheres resulted in a 16% decrease in collagen content in a scar model when compared with untreated animals (p less than 0.05). With further refinement of this method of drug delivery, clinically useful inhibition of scar formation may result.

Pharmacokinetics and metabolism of 14C-oxaceprol in beagle dogs after intramuscular and oral administration

Blood and plasma levels and renal and faecal excretion of 14C-oxaceprol (N-acetyl-4-hydroxyproline, AHP 200) were measured after p.o. and after i.m. administration to 3 male Beagle dogs each. In addition, the concentration of 14C-oxaceprol equivalents in synovia was determined at tmax and at tmax + t1/2(elim.). The pattern of metabolites was investigated for plasma, synovia, and urine samples. Peak plasma levels were obtained for the individual animals between 20 and 30 min after i.m. administration and between 2 and 2.5 h after p.o. administration, respectively. The concentrations of 14C-oxaceprol equivalents in synovia 2.5 and 3.5 h after oral administration and 1.3 h after i.m. administration were comparable to the respective plasma levels at the same time. After oral administration the amount of absorbed oxaceprol varied between 43 and 79% of the dose. Oxaceprol was exclusively eliminated via the kidneys. Excretion was complete within 72 h p.a. for both administration routes. Elimination half-lives were 44 +/- 9 min. or 68 +/- 14 min. (means +/- S.D.) after i.m. and p.o. administration, respectively. Oxaceprol was not metabolized.

Biological variations in serum total hydroxyproline concentration in the beagle dog

A newly developed assay for hydroxyproline (Hyp) in physiological samples was used for determining the biological variations in serum total Hyp in the beagle dog. The results showed that dietary Hyp restriction in the beagle dog results in a significant decrease in serum total Hyp over the first 24 h and that there are no obvious circadian variations in serum total Hyp concentrations in beagle dogs on dietary Hyp restriction.

C1q content of serum and lung lavage fluid of rats exposed to toxic levels of oxygen

We used a sensitive hemolytic assay to measure the level of C1q, a subcomponent of the first complement protein in the classic pathway, in bronchoalveolar lavage fluid and serum of rats exposed to air and hyperoxia. The serum level was 125 +/- 5 micrograms/ml and the lavage level was 41 +/- 17 ng/ml in rats breathing air. In rats exposed to acute oxygen toxicity (95% O2 for 66 hr), the serum level was at 107 +/- 10 micrograms/ml, but the level in lavage fluid increased to 2457 +/- 400 ng/ml (p less than 0.05 compared to air). Administration of the proline analogue cis-4-hydroxy-L-proline to air- and O2-exposed rats reduced the serum C1q level by 28% and 34%, respectively (both p less than 0.05), presumably by interfering with metabolism of the collagen-like sequence of C1q. The level of C1q in bronchoalveolar lavage fluid is a sensitive marker of acute lung injury.

Different tissue uptake after administration of proline and hydroxyproline in neonatal piglets

Using whole body autoradiography and impulse counting technique, distribution of intramuscularly administered ¹⁴C-DL-hydroxyproline and ¹⁴C-L-proline in newborn piglets was compared. The main excretion route of hydroxy-proline was via the kidneys. Hydroxyproline or its metabolic products were excreted at quite another rate by the liver than was proline. Skeletal muscles and myocardium showed a lowered ratio of proline to hydroxyproline. A difference in favour of hydroxyproline in passing the blood-brain barrier was shown. Contrary to proline hydroxyproline yielded a low ¹⁴C-level in the skeletal tissues. The possibilities of an incomplete hydroxylation of proline as an aetiological factor in the origin of skeletal disorders are discussed.