Control of thyroid cell proliferation and goitrogenesis

Human thyroid cells proliferate during development and in adults in response to physiologic and pathologic stimuli. Under normal conditions, they turn over about once every 8 years. The main physiologic regulators are thyrotropin and iodide and, in disease, thyroid-stimulating and thyroid-blocking antibodies. Growth factors modulate proliferation in vitro, but their role in vivo is still unknown. Mitogenic effects are mediated via three major pathways: the cyclic AMP, protein tyrosine kinase, and the Ca(2+) phosphatidylinositol cascades. In this review, the role of these cascades in hyperthyroidism, congenital thyroid defects, and autonomous adenoma is analyzed.

The TSH cyclic AMP cascade in the control of thyroid cell proliferation: the story of a concept

Thyrotropin stimulates the growth and proliferation of thyroid cells in vivo. Starting in the 1970, we have progressively shown that these effects could be reproduced in vitro in dog thyroid cells in primary culture. They are accompanied by the expression of differentiation. All the effects of thyrotropin are mediated by the cyclic AMP cascade. The best argument that this concept applies in vivo is the generation of hyperfunctioning adenoma involving the whole gland in transgenic mice expressing the constitutively active adenosine A2 receptor in the thyroid.

Intermediate filaments in normal thyrocytes: modulation of vimentin expression in primary cultures

In dog thyrocyte primary cultures, the antagonistic effects of thyrotropin (TSH) and epidermal growth factor (EGF) on differentiation expression were accompagnied by distinct long-term morphological changes: TSH-treated cells showed an epitheloid morphology; EGF reversibly induced a fusiform shape. Using indirect immunofluorescence microscopy and two-dimensional gel electrophoresis, we studied the modifications in the distribution and synthesis of the intermediate filament proteins of the cytoskeleton in response to TSH and EGF. These factors had little effect on the expression of cytokeratins 8 and 18, which were expressed in 98% of cells. However, TSH induced a profound redistribution of cytokeratins (and actin) with the appearance of a marked staining of cell junctions. Vimentin was coexpressed with cytokeratins in about 40% of cells from normal thyroid follicles freshly isolated by collagenase. During culture, immunostained vimentin network progressively developed in 90% of control and EGF-treated cells simultaneously with vimentin synthesis. In contrast, only 20% of TSH-treated cells reacted with vimentin antibody and we observed a marked decrease in vimentin synthesis in response to TSH. Therefore, vimentin synthesis, which should occur in at least some normal thyroid follicles in vivo, was inhibited in vitro by TSH which promotes differentiation expression. However, EGF-treated cells thereafter cultured with TSH regained an epitheloid morphology and differentiation in spite of the persistency of a complete network of vimentin.

Reduction of Ib autogenetic inhibition in motoneurons during contractions of an ankle extensor muscle in the cat

1. Triceps surae and plantaris (Pl) motoneurons were recorded intracellularly in chloralose or pentobarbital sodium (Nembutal)-anesthetized cats during unfused tetanic contractions of gastrocnemius medialis muscle (GM) produced by stimulating either a cut branch of the GM nerve or the muscle directly. 2. In alpha-motoneurons, during a series of GM twitches at 10/s, contraction-induced inhibitory potentials, probably the result of input from Golgi tendon organs (autogenetic inhibition), rapidly subsided before the end of the series. In contrast, excitatory potentials, probably the result of the activation of spindle primary endings during relaxation from contraction, persisted. 3. In gastrocnemius lateralis-soleus (GL-S) and Pl motoneurons lacking an excitatory connection with Ia afferents from GM, the sustained contraction of this muscle also elicited a declining inhibition. Rapid reduction of contraction-induced autogenetic inhibition was also observed in homonymous gamma-motoneurons. During unfused tetanic contractions lasting 0.5-4s, inhibitory potentials quickly subsided, but an abrupt increase in contractile force elicited a new series of decreasing inhibitory potentials. 4. The assumption that the inhibition induced by GM unfused tetanic contractions was due to activation of homonymous Ib afferents was supported by observations of the effects of electrical stimulation of the GM nerve. In Pl motoneurons lacking an excitatory connection with Ia afferents from GM, repetitive trains applied to the GM nerve, at a strength just above threshold for group I fibers, elicited rapidly declining inhibitory potentials similar to those produced by GM contraction. It was verified that during such stimulation, the amplitude of the group I afferent volleys did not decrease. 5. Reduction of contraction-induced Ib inhibition during sustained GM contraction was still present after a low spinalization of the preparation. As GM tendon organ discharges were verified to persist throughout prolonged contractions, the observed decline of autogenetic inhibition is likely to depend on a spinal mechanism, possibly involving presynaptic inhibition of Ib afferents and/or mutual inhibition of Ib-inhibitory interneurons.

Control of protein synthesis by thyrotropin and epidermal growth factor in human thyrocytes: role of morphological changes

The effect of thyrotropin (TSH) and epidermal growth factor (EGF) on the synthesis of proteins has been studied using two-dimensional gel electrophoresis in primary cultures of thyroid cells developing as a monolayer or that remained associated as dense aggregates. (1) A 4-day treatment of monolayer cells by TSH or dibutyryl cAMP enhanced the synthesis of 26 proteins and decreased that of 19 others. (2) The synthesis of 29 proteins was similarly modified by TSH and dibutyryl cAMP in both types of culture organizations. Both agents stimulated the synthesis of thyroperoxidase and of proliferating cell nuclear antigen (PCNA)/cyclin and decreased that of actin and of a high Mr isoform of tropomyosin. (3) TSH induced the retraction of monolayer cells. Its effect on the synthesis of many proteins was mimicked by culturing unstimulated cells as dense aggregates instead of monolayers which similarly affected cell morphology. (4) EGF alone had no effect on protein synthesis in monolayer cells but it inhibited both the morphological changes induced by TSH and dibutyryl cAMP and the effect of these agents on the synthesis of 23 proteins including thyroperoxidase.

IN CONCLUSION

(1) TSH and cAMP induce both proliferation and the expression of differentiation in thyroid cells while EGF has a small mitogenic effect but a marked inhibitory action on differentiation expression; (2) many TSH (cAMP) and EGF effects on the pattern of protein synthesis might be related to morphological changes; (3) the expression of the differentiation marker thyroperoxidase and of the mitogenic marker PCNA/cyclin appears independent of cell configuration and morphology.

Use of two-dimensional gel electrophoresis and autoradiography as a tool in cell biology: the example of the thyroid and the liver

Different applications of two-dimensional gel electrophoresis and the research strategies that this methodology allows, with examples drawn from our own work on thyroid and liver cells, are described.

Detection of bovine leukemia virus antibodies in bulk tank milk using an ELISA test: improvement of the predictive value of results by repeated testing

In 9457 dairy farms located in an area with low prevalence of bovine leukemia virus (BLV) infection, bulk tank milk was examined to detect for the presence of antibodies using an ELISA test. If the result was positive or doubtful, serum of all animals in the farm was tested and bulk tank milk was tested again five times every 8-12 days. The results were used to establish decision rules in the event of a positive or doubtful result during mass screening.

Actin stress fiber disruption and tropomyosin isoform switching in normal thyroid epithelial cells stimulated by thyrotropin and phorbol esters

Thyrotropin (TSH), through cyclic AMP, promotes both proliferation and differentiation expression in dog thyroid epithelial cells in primary culture, whereas the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) also stimulates proliferation but antagonizes differentiating effects of TSH. In this study, within 20 min both factors triggered the disruption of actin-containing stress fibers. This process preceded distinct morphological changes: cytoplasmic retraction and arborization in response to TSH and cyclic AMP, cell shape distortion, and increased motility in response to TPA and cyclic AMP, cell shape distortion, and increased motility in response to TPA and diacylglycerol. TSH and TPA also induced a marked decrease in the synthesis of three high Mr tropomyosin isoforms, which were not present in dog thyroid tissue but appeared in culture during cell spreading and stress fiber formation. In contrast, the synthesis of two low Mr forms of tropomyosin that were already present in thyroid tissue remained unchanged after treatment with TSH or TPA. Epidermal growth factor, another mitogenic and dedifferentiating factor for these cells, did not induce acute morphological changes, nor modification of tropomyosin synthesis. The tropomyosin isoform switching observed here closely resembled similar processes in various cells transformed by oncogenic viruses. However, it did not correlate with differentiation or mitogenic activation. Contrasting with current hypothesis on this process in transformed cells, tropomyosin isoform switching in normal thyroid cells was preceded and thus might be caused by early disruption of stress fibers.

Protein synthesis during induction of DNA replication in thyroid epithelial cells: evidence for late markers of distinct mitogenic pathways

The synthesis of specific protein has been investigated in primary cultures of dog thyroid epithelial cells, which can be induced to progress into G1 phase, in the presence of insulin, by different types of mitogens: thyrotropin (TSH) acting through cyclic adenosine monophosphate (cAMP), epidermal growth factor (EGF), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), or 10% serum. EGF, TPA, or serum specifically induce [35S] methionine labeling of protein 1 (Mr approximately 80,000). The effect of EGF on protein 1 labeling and DNA replication is dependent on insulin. The level of protein 1 labeling as well as that of DNA synthesis is higher when TSH or TSH + serum are added together with EGF. It peaks in mid-G1. TSH alone, in the presence of insulin, stimulates DNA replication without inducing protein 1 synthesis, which thus represents a cell-cycle-dependent event that is not obligatory in mitogenic activation through cyclic AMP. Among the eight proteins whose synthesis is stimulated by TSH, only the labeling of protein 7, molecular weight ratio (Mr approximately 38,000), correlates with the DNA synthetic activity of the cells. The present authors identified protein 7 as cyclin/proliferating cell nuclear antigen (PCNA), the auxiliary protein of DNA polymerase-delta. The effect of TSH on cyclin synthesis is already detectable when most of the cells are in late G1, but its stimulation by EGF or EGF + serum is delayed and detected only after extending the labeling period to the S-phase. These data support the view that the cAMP-mediated mitogenic pathway remains partly distinct from the better known pathways induced by growth factors and tumor promoters, even at late stages of the G1-phase.

The degradation of Indulin ATR by Streptomyces: chemical characterization of the water soluble acid precipitable products

Two strains of Streptomyces, S. badius and S. virdosporus, when incubated with kraft lignin (Indulin ATR) produce water soluble acid precipitable compounds. We show by a combination of elemental analysis, amino acid analysis, and FTIR that these are complexes of lignin fragments strongly associated with proteins. The latter, which can be separated by solvent extraction, are added by the microorganisms themselves. The lignin fraction, which represents approximately 60% of the complex weight, consists of lower molecular weight, lower methoxyl content Indulin fragments. KeyWords: lignin, Indulin, lignin–protein complexes, Fourier transform infrared spectrophotometry, high performance size exclusion chromatography, Streptomyces.

Cloning and sequencing of a calcium-binding protein regulated by cyclic AMP in the thyroid

p24 is a thyroid protein (Mr 24,000) identified by two-dimensional gel electrophoresis on the basis that its synthesis and phosphorylation are up-regulated by thyrotropin and cyclic AMP agonists. p24 cDNA was cloned from a lambda gt11 cDNA library using a polyclonal antibody raised against the protein recovered from a Western blot spot. The encoded polypeptide (189 residues) displays a putative target-site for phosphorylation by cyclic AMP-dependent protein kinase and belongs to the superfamily of proteins binding Ca2+ through 'EF hand' domains. It presents four such domains of which two agree closely with the consensus. The ability of p24 to bind Ca2+ has been directly confirmed on Western blots. p24 was detected in many tissues including the salivary glands, the lung and the brain. The ubiquitous nature of p24, together with its regulatory and sequence characteristics suggest that it constitutes an important target common to the cyclic AMP and Ca2+-phosphatidylinositol cascades.

Effects of a selenium deficient diet on thyroid function of normal and perchlorate treated rats

Pregnant rats were submitted to a selenium-deficient diet immediately after mating; it was continued for 4 weeks after delivery. The pups were sacrificed at 3 and 4 weeks of age. Perchlorate, an antithyroid agent inhibiting iodide trapping in the thyroid, was administered via the drinking water to half of the rats. Rats submitted to a normal laboratory diet and to the experimental diet supplemented with selenium were used as controls. The effects of selenium deficiency were an increase in the number of growth abnormalities, growth retardation, and decreased seleno-dependent glutathione peroxidase (GSH-Px) activity in plasma and in various organs. These effects were relieved by selenium supplementation in the diet. Perchlorate treatment induced the classic picture of primary hypothyroidism. Selenium deficiency increased thyroid hormone levels in perchlorate-treated rats and in controls drinking tap water. In the latter group, it also decreased TSH plasma concentration and thyroid weight. These effects were partially reversed by Se supplementation. In vitro experiments, performed on adult rats, revealed increased radioiodide uptake and organification in glands from the rats submitted to the selenium-free diet. Plasma T3 half-life was similar in control and Se-deficient rats. These data suggest a higher efficiency of thyroid hormone synthesis in the thyroids of selenium-deficient rats, despite a lower thyroid stimulation as evaluated by serum TSH. They are compatible with the hypothesis that decreased selenium supply, leading to a decreased GSH-Px in the thyroid, increases hydrogen peroxide steady state level and thus thyroid peroxidase activity and thyroid hormone synthesis.

Differential protein phosphorylation in induction of thyroid cell proliferation by thyrotropin, epidermal growth factor, or phorbol ester

Protein phosphorylation was studied in primary cultures of thyroid epithelial cells after the addition of different mitogens: thyrotropin (TSH) acting through cyclic AMP, epidermal growth factor (EGF), or 12-O-tetradecanoylphorbol-13-acetate (TPA). EGF or TPA increased the phosphorylation of five common polypeptides. Among these, two 42-kilodalton proteins contained phosphotyrosine and phosphoserine with or without phosphothreonine. Their characteristics suggested that they are similar to the two 42-kilodalton target proteins for tyrosine protein phosphorylation demonstrated in fibroblasts in response to mitogens. No common phosphorylated proteins were detected in TSH-treated cells and in EGF- or TPA-treated cells. The differences in the protein phosphorylation patterns in response to TSH, EGF, and TPA suggested that the newly emerging cyclic AMP-mediated mitogenic pathway is distinct from the better known growth factor- and tumor promoter-induced pathways.

Pathogenesis of autonomous thyroid nodules: in vitro study of iodine and adenosine 3',5'-monophosphate metabolism

The in vitro characteristics of iodide and cAMP metabolism have been compared in tissues from autonomously functioning thyroid nodules and their quiescent counterpart to test the hypothesis that autonomy may result from constitutive activation of the tissue's TSH, cAMP, and protein phosphorylation regulatory axis, as in vivo nodular tissue took up more iodide. This effect was entirely due to increased transport capacity, the affinity of iodide transport, and the fractional binding of iodide to protein remaining unchanged. However, at high concentrations total iodide binding to protein was similar in quiescent and nodular tissue. In both tissues, this metabolic step was enhanced by phorbol esters and the ionophore A23187. As evaluated by autoradiography of two-dimensional gel protein electrophoregrams, no differences in the patterns of protein synthesis or phosphorylation between quiescent and nodular tissue were found. Basal cAMP levels were similar in quiescent and nodular tissue. The cAMP response to TSH was lower in nodular tissue, with no change in sensitivity or kinetics; both tissues responded to forskolin. No systematic suppression of iodide inhibition or abnormal responses to other hormones or neurotransmitters were found. Three proteins (24K-1, 24K-2, and 26K) were phosphorylated only in the presence of TSH or forskolin in both quiescent and nodular tissue. One protein substrate (20K) was phosphorylated in the presence of TSH in the quiescent, but not in the nodular, tissue. In conclusion, 1) slices from autonomous thyroid nodules reproduce the in vivo characteristics of the lesion and are, therefore, a suitable in vitro experimental model for biochemical studies; 2) taken together with data from transplantation experiments, the reproduction in vitro or its in vivo characteristics suggest an inherent defect in the nodule; 3) the homogeneity of biochemical findings within each nodule is compatible with the clonality of the lesion; 4) the autonomous nodule is a minimal deviation tumor; and 5) the characteristics of the TSH, cAMP, protein phosphorylation cascade are qualitatively normal, and autonomy does not result from constitutive activation of this system; and 6) a 20K protein, not phosphorylated in response to TSH in the nodule, could represent an absent negative controlling element.

Use of electroblotting to detect and analyze phosphotyrosine containing peptides separated by two-dimensional gel electrophoresis

A technique to detect and analyze phosphotyrosine containing peptides after separation of total cellular proteins by two-dimensional gel electrophoresis is described. This is achieved by electroblotting of proteins on nylon membranes followed by alkali treatment. In comparison with direct alkali treatment of the polyacrylamide gel, this procedure is easier to perform; avoids the diffusion of proteins out of the gel during alkali treatment; allows a more precise localization of phosphotyrosine containing peptides on the untreated membrane; and is less time consuming with respect to extraction of proteins for phosphoamino acid analysis.

Photolysis and ozonolysis of (iso)desmosine-containing crosslinked peptides from porcine aorta elastin

This report describes the use of photolysis and ozonolysis as a means of achieving complete cleavage of the pyridinium ring of (iso)desmosine in crosslinked elastin peptides. Although photolysis leads to the opening of the ring with concomitant formation of lysine, the peptide chains remain attached. Subsequent ozonolysis is able to completely achieve the cleavage of the rest of the ring skeleton, thus leading to the separation of the peptide chains. Formation of new amino acids, i.e. alpha-aminoadipic and glutamic acids, is emphasized. Localization of these amino acids within the released peptides should be of help in structural investigations on the crosslinking zones involving either isodesmosine or desmosine. However, other amino acids such as tyrosine and phenylalanine are sensitive to this procedure and side reactions occur which are responsible for peptide bond cleavage with the formation of breakdown products.

Determination of proteins and sulfobetaine with the Folin-phenol reagent

This paper describes a method for the quantitative analysis of solutions containing a mixture of proteins and sulfobetaine. In a preliminary step the proteins, which interfere with the detergent assay, are separated by precipitation with trichloroacetic acid (8%). The insoluble fraction, dissolved in NaOH (1.0 N), and the soluble fraction, containing the detergent, are treated with the Folin-Ciocalteu phenol reagent, essentially following the method of O. H. Lowry, N. J. Rosebrough, A. L. Farr, and R. J. Randall (1951, J. Biol. Chem. 193, 265-275). The absorbance of the protein fraction is read, as usual at 750 nm, while that of the detergent solution is read at 342 nm. At this wavelength, sulfobetaine, treated with the Folin reagent, absorbs strongly, the absorbances being proportional to its concentration up to 1.5 mg/ml.

Differential protein synthesis in the induction of thyroid cell proliferation by thyrotropin, epidermal growth factor or serum

Protein synthesis in the G1 period of the cell cycle has been investigated using two-dimensional gel electrophoresis in primary cultures of dog quiescent thyroid cells, incubated in defined medium and induced to proliferate by the combined action of thyrotropin (TSH), epidermal growth factor (EGF) and serum or by each of these agents, acting alone. The analysis of the proteins, pulse-labeled for 3 h with [35S]methionine, in quiescent cells deprived of serum and in cells that had been stimulated for various periods of time by the addition of TSH, EGF and serum showed maximal modifications before entry into S phase: the labeling of at least ten proteins was enhanced while that of at least six proteins was decreased. The synthesis of one of these proteins (protein 1; Mr approximately equal to 81 000) was maximal 9-12 h after stimulation by the proliferative agents but began to decrease at 15-18 h and was still decreased at 29-32 h. The study of the effect of each of the proliferation agents alone on the labeling of these sixteen proteins showed that TSH specifically stimulated the labeling of eight polypeptides (proteins 2-9) and that, in contrast, EGF and serum specifically increased the labeling of two other proteins (proteins 1 and 10). The labeling of one protein was decreased by each of the different agents (protein 6') while TSH specifically decreased the labeling of four polypeptides (proteins 1'-4') and increased the labeling of one polypeptide (protein 5') whose synthesis was decreased by EGF and serum. The specific effect of TSH on one protein labeling (protein 7; Mr approximately equal to 39 000) was potentiated by EGF and serum while the specific effect of EGF and serum on another protein labeling (protein 1) was potentiated by TSH. There is thus a correlation between the level of synthesis of these two proteins and the proliferative state of the cells, which is much greater when the stimulating agents are acting together. The induction of protein 1 synthesis by EGF was no longer observed when the cells were no longer proliferating. In the same way, TSH no longer stimulated the synthesis of protein 7 in thyroid cells at confluence. In conclusion, the present study has identified some proteins (proteins 1 and 7) which, as judged by the peculiar stimulation and the kinetics of their synthesis, could be part of the final key events triggering DNA replication in thyroid cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of proteins synthesized by fibroblasts from patients with cystic fibrosis by two-dimensional gel electrophoresis and double label autoradiography

Mucoviscidosis, the most frequently lethal genetic syndrome of Caucasian population, is a recessive disease with multiple tissue involvement. Although the major pathological changes are observed in lungs and pancreas, abnormalities have also been detected in several other exocrine glands. For many reasons, such as the ready availability of tissue material, the absence of secondary changes and the potential for prenatal diagnosis, cultured skin fibroblasts could be the tissue of choice to search for the primary defect. Several abnormalities have been reported in CF fibroblasts, suggesting that the genetic abnormality is expressed in these cells. To search for potentially mutant protein(s) we have compared the protein composition of normal and CF fibroblasts by two dimensional gel electrophoresis and double-labeling autoradiography using 35S and 75Se methionine as tracer. The results demonstrate the power of the method; however, we have not found one protein spot consistently missing in CF cells. Possible reasons for the absence of a single common identifiable defect are discussed.

Thyrotropin modifies the synthesis of actin and other proteins during thyroid cell culture

Primary cultures of dog thyroid cells have been used to study the effects of thyrotropin on the synthesis of proteins. The cells were cultured for 4 days in serum-free and thyrotropin-free conditions. Thyrotropin was then added for varying periods of time (6-96 h). In the absence of thyrotropin, the cells have an elongated flattened aspect. Exposure to thyrotropin for 6-24 h produces retraction and rounding up of cells whereas cells incubated with thyrotropin for longer periods of time have an epithelial cuboidal shape. After varying periods of culture the cells were labelled with [35S]methionine for 6 h and then analyzed by one- and two-dimensional gel electrophoresis, followed by autoradiography. The results were as follows. After exposure to thyrotropin for 32 h and 48 h, the synthesis of about 18 proteins was increased while that of about 14 others was decreased. After 6 h the labelling of three and five of these proteins was already increased or decreased, respectively. Some of the proteins whose synthesis is modified in the presence of thyrotropin were identified. Actin synthesis was markedly decreased with a maximum 24-48 h after the addition of thyrotropin. A modification in the ratio between alpha and beta tubulins was also observed together with very large changes in a group of proteins having both the relative molecular mass (30 000-40 000) and the isoelectric points of tropomyosins. Forskolin and cholera toxin caused the same qualitative and quantitative changes as thyrotropin; this suggests that the regulation by thyrotropin of the synthesis of several thyroid cell proteins is mediated by cAMP. In conclusion, the data obtained in this work might help to explain the molecular mechanisms by which thyrotropin (and cAMP) triggers the changes in cell shape which occur during thyroid cell culture. They also indicate that one of the main effects of thyrotropin takes place at the level of several proteins which belong to the cytoskeleton and which are involved in the definition of the cytostructure of the thyroid cells.

Photolysis and ozonolysis of desmosine and elastolytic peptides

This paper describes the combined effects of photolysis and ozonolysis upon the main interchain crosslinks of elastin, desmosine and isodesmosine. Photolysis of purified (iso)desmosines in solution leads to the cleavage of the pyridinium rings to give lysine and an analogue of glutaconic aldehyde which is deformylated to a stable compound absorbing at 242 nm. This photoproduct is subsequently fragmented by ozone into glutamic, alpha-aminoadipic and possibly alpha-amino-delta-oxocaproic acids. However the yields of these different compounds are very low because we observed that numerous competing side reactions (polymerisation, recyclisation) accompany the photoozonolytic decomposition of the pyridinium rings of free (iso)desmosines. The results are clearer when reticulated elastolytic peptides are photoozonolysed. The (iso)desmosines, covalently linked in these peptides, are cleaved into lysine, glutamic and alpha-aminoadipic acids (in a ratio 2:1) with yields corresponding to 80-90% of those expected from the decomposition of the (iso)desmosines originally present in the peptide fraction. We have also observed that ozonolysis alone degraded another crosslink present in these peptides, the aldol condensation product, resulting in the production again of glutamic and alpha-aminoadipic acids in amounts consistent with the known concentrations of this particular crosslink in elastin. Finally we noted that the complete photoozonolytic degradation of the (iso)desmosines present in a semi purified reticulated elastolytic fraction resulted in a shift of the size distribution of these peptides toward lower values. It is not certain however that this shift, indicative of a freeing of the polypeptide chains from their original three dimensional network, is due uniquely to the cleavage of the (iso)desmosines. Indeed we have observed that tyrosine and phenylalanine were also degraded during photoozonolysis. Not knowing the mechanism of this degradation it is impossible to rule out the possibility that concomitant cleavages of peptide bonds did occur.