Fetal Cartilage Xylosyltransferase Activity and Skeletal Growth in Sheep

0031-3998/85/19 12-1240$02.00/0 PEDIATRIC RESEARCH Vol. 19, No. 12, 1985 Copyright 63 1985 International Pediatric Research Foundation, Inc. Printed in U.S. A. Fetal Cartilage xylosyltransferase Activity and Skeletal Growth in Sheep FRANK H. MORRISS, JR., BRIAN FITZGERALD, AND LAVON M. RIDDLE Houston Perinatal Nutrition Laboratory, Department ofpediatrics, Medical School, University of Texas Health Science Center at Houston, Houston, Texas 77030 ABSTRAa. The activity of UDP-D-xy1ose:proteoglycan predominantly a Dorset-Rambouillet cross. The ewes were of core protein B-D-xylosyltransferase(EC 2.4.2.26), the en- known gestational age (65 to 138 days; term 147 days) that was zyme that catalyzes the initiation of the polysaccharide established by breeding history and confirmed by fetal radi- chain linkage to the core protein of proteoglycans, was ographs. The ewes were sacrificed by exsanguination or lethal measured in costal cartilage from 20 fetal sheep of 65-138 injection. Each fetus was dried and weighed, the vertebral column days gestation. Activity of the enzyme was estimated from length was measured, and the anterior chest wall of the fetus was the transfer of [14Cjxylosefrom UDP-[14Cjxyloseto silk as removed and stored at -70" C for no longer than 12 months the acceptor protein. The specific activity decreased ap- before further enzyme preparation was done. The anterior chest proximately 10-fold and was found to be highly correlated wall was thawed, and the cartilaginous portions (not including with the decremental rate of growth in length of the fetal the growth plate) of the sixth through the ninth ribs were cleaned vertebral column. These observations, together with the free of tissue while wetted with a buffer which was 0.05 M KC1, known gestational decrease in the in vitro rate of uptake 0.05 M 2(N-morpholino)etanesulfonic acid at pH 6.5, containing of radiolabeled sulfate by ovine fetal cartilage, a subsequent 0.006 M MnC12 and 0.012 M MgC12 at 4" C. The cartilage was step in proteoglycan synthesis, support the hypothesis that chopped into small pieces and shredded with a Tekmar Tissue- normal fetal skeletal growth is dependent during the last mizer (Tekmar Co., Cincinnati, OH) for 9 min in 10 ml of the one-half of gestation on the activity of xylosyltransferase above buffer during which process the cartilage was cooled to 4" in cartilage. (Pediatr Res 19: 1240-1243,1985) C at 3-min intervals. Next, the tissue suspension was homoge- nized by hand with 20 strokes of a Tenbroeck homogenizer while kept ice cold. The homogenate was centrifuged at 10,000 x g for 10 min at 4" C; the supernate was used as the enzyme solution In the synthesis of proteoglycans by chondrocytes, sulfation is in the xylosyltransferase assay. The 10,000 x g supernate may a late step (1). In the sheep fetus the diminishing fractional contain endogenous acceptor proteins in low concentration. growth rate of the skeleton during gestation is paralleled by Preparation of acceptor protein. The xylosyltransferase activity decreasing uptake of radiolabeled sulfate by costal cartilage in was assayed by the rate of transfer of xylose from uniformly vitro (2). Although fetal serum sulfate concentration also de- labeled UDP-['4C]xyloseto an acceptor protein (5). The protein creases during the course of gestation, fetal cartilage sulfate acceptor for the transfer of xylose to connective tissue protein uptake for incorporation into proteoglycans is not limited by catalyzed by the enzyme appears to require a glycine residue on sulfate availability (2). Hence, a rate-limiting step in the synthesis the C-terminal side of a xylose-accepting serine residue; silk of proteoglycans by fetal cartilage may occur prior to the final contains a large number of Ser-Gly-sequences, and thus serves sulfation step. A possibly critical step in the synthesis of proteo- as an acceptor of high activity (6). We used silk as the acceptor glycans is that which initiates polysaccharide chain formation by protein; use of silk as the acceptor protein for the activity of transfer of D-xylose from UDP-D-xylose to the protein core of xylosyltransferase obtained from chick embryo cartilage recently the proteoglycan, a reaction that is catalyzed by UDP-D-xy- has been described (6). Silk worm cocoons were obtained from 1ose:proteoglycan core protein P-~xylosyltransferase (EC the International Silk Association, USA (New York, NY). These 2.4.2.26) (1, 3, 4). were sliced open, and the worms were removed. The cocoons We conducted this survey of fetal sheep costal cartilage xylo- were cut into small pieces and boiled in distilled water until the syltransferase specific activity to determine if enzyme activity cocoon pieces had unraveled into strands; this usually required decreases during gestation. We also sought to determine if a boiling for 2 days or less. The strands were lyophilized to dryness, decrease in enzyme specific activity correlated with the decre- and the dry silk was dissolved in aqueous 60% (wtlvol) LiSCN mental fractional growth rate of the fetal skeleton during the last (Alfa Chemical Co., Danvers, MA) at a ratio of 1 g silk: 10 ml half of gestation, a correlation that would support the hypothesis LiSCN. This solution was centrifuged at 1500 x g for 15 min to that the rate of fetal skeletal growth is dependent on the activity obtain a supernate containing the dissolved silk; to this were of xylosyltransferase in cartilage. added 90 ml of 0.25 M EDTA at pH 6.2. The silk-EDTA solution was dialyzed in Spectrapor 3 dialysis tubing (Spectrum Medical MATERIALS AND METHODS Industries, Inc., Los Angeles, CA) against flowing deionized water (250 ml/h) for 48 h or until the effluent gave a negative test for Extraction of cartilage enzyme activity. Twenty fetal sheep SCN- with FeCl3. The dialized silk solution was centrifuged at were removed from the uteruses of range-fed western ewes, 1500 x g for 15 min, and the supernate was kept at 4" C for no longer than 2 days before being used as the acceptor protein in Received May 15, 1985; accepted July 16, 1985. the xylosyltransferase assay. Address for reprints and correspondence Frank H. Morriss, Jr., M.D., Depart- ment of Pediatrics, University of Texas Medical School at Houston, 6431 Fannin Preparation of substrate. A solution that was 0.015 M KCl, Street, Houston, TX 77030. 0.0165 M 2(N-morpholino)ethanesulfonic acid at pH 6.5, con- Supported by HHS Grant HD-11337. taining 10 pCi/ml of UDP-['4C]xylose (267 mCi/mmol, New 40 XY LOSY LTRANSFERASE ACTIVITY AND GROWTH England Nuclear, Boston, MA) and 0.24 pmol/ml of unlabeled UDP-xylose (Sigma Chemicals, St. Louis, MO) was used as the substrate solution. Enzyme assay. The assay for xylosyltransferase activity was identical to that described by Roden et al. (5), except that the 1% silk solution described above served as the protein acceptor, and that 10 ml Hydrofluor (National Diagnostics, Somerville, NJ) was used instead of the described scintillation mixture. The 50 p1 of the enzyme solution, 20 p1 of the acceptor solution, and 5 p1 of the UDP-['4C]xylose substrate solution were incubated for 40 min then stopped by placement on ice and the addition of 10 p1 of 0.25 M EDTA at pH 8.0. Control incubation mixtures included the enzyme, acceptor protein, and substrate solutions, but were prepared with 10 p1 0.25 M EDTA at pH 8.0 prior to incubation for 40 min on ice. To assay xylose incorporation, 0.05 ml of 1% bovine serum albumin was added to the incubation mixture, proteins were precipitated by the addition of 0.15 ml of 10% trichloroacetic acid-4% phosphotungstic acid, col- lected by centrifugation, washed twice with 0.3 ml of 5% trichloroacetic acid, and dissolved in 0.1 ml of 1 M NaOH. Each sample was transferred to a scintillation vial, diluted to 0.4 ml with water, and 0.1 ml of 1 M acetic acid was added to neutralize the NaOH before the radioactivity was measured. Dpm in the final reaction product were converted to total ng xylose transferred/ mg cartilage protein by considering both the labeled and unlabeled xylose in the incubation mixture. Cartilage protein was assayed by a modification of the Bradford method (7, 8). Each fetal cartilage assay was performed in triplicate; the coefficient of variation of three replicates was 4%. Statistics. Linear regression equations were calculated by the least squares method, and the significance of the slope of the equation was tested with the F ratio test. GESTATIONAL AGE, days Fig. 2. Top, fetal sheep vertebral column length as a function of gestational duration. The increase in length from mid-gestation to term may be expressed by the equation: Y = -13 + 0.39X (r = 0.98; p c 0.0001). Bottom, fractional increase in length of fetal sheep vertebral column as a function of gestational duration. The diminishing growth rate from midgestation to term may be expressed by the equation: Y = 0.39/(- 13 + 0.39X). RESULTS Fetal sheep costal cartilage xylosyltransferase specific activity decreased approximately 10-fold as a function of gestational duration (Fig. 1). This relationship may be expressed by the linear regression equation: Y = 204 - 1.45 X, where Y is ng xylose transferred to the silk acceptor protein per mg cartilage protein after 40 min incubation at 37" C, and X is gestational duration in days. In Figure 2 (top) is shown the vertebral column length as a function of gestational age. Vertebral column length increased 4-fold during the interval of gestation surveyed.

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