Biosynthesis of cyanogenic glucosides: in vitro analysis of the glucosylation step

The last step in the biosynthesis of cyanogenic glucosides, the glucosylation of the cyanohydrin intermediate, has been investigated in detail using Triglochin maritima seedlings. The glucosyltransferase activity is not associated with membranes and appears to be a "soluble" enzyme. The cyanohydrin intermediate, which is formed by hydroxylation of 4-hydroxyphenylacetonitrile by a membrane-bound enzyme, is free to equilibrate in the presence of the glucosyltransferase and UDPG, because it can be trapped very efficiently. This indicates that this intermediate is not channeled (unlike some of the other intermediates), although it is probably the most labile of all of them. The glucosyltransferase of T. maritima responsible for the glucosylation of the cyanohydrin was separated from another glucosyltransferase, which used 4-hydroxybenzylalcohol as a substrate, and purified over 200-fold. It catalyzed the glucose transfer from UDPG to only 4-hydroxymandelonitrile and 3,4-dihydroxymandelonitrile, giving rise to the respective cyanogenic glucosides. Although the activities with these two substrates behaved differently in certain respects (e.g., extent of inactivation during purification and difference in activation by higher salt concentrations), most of the data acquired favor the view that only one enzyme in T. maritima is responsible for the glucosylation of both substrates.     

Binding of simple carbohydrates and some N-acetyllactosamine-containing oligosaccharides to Erythrina cristagalli agglutinin as followed with a fluorescent indicator ligand

Erythrina cristagalli agglutinin, a dimeric lectin [J. L. Iglesias, et al. (1982) Eur. J. Biochem.123, 247–252] was shown by equilibrium dialysis to be bivalent for 4-methylumbelliferyl-β-d-galactoside. Upon binding to the lectin, this ligand showed a difference absorption spectrum with two maxima (at 322 and 336 nm) of equal intensity (Δ? = 1.2 × 10³m^?1 cm^?1). A similar spectrum with a comparable value of Δ? was obtained with 4-methylumbelliferyl-N-acetyl-β-d-galactosaminide. Binding of methyl-α-d-galactoside, lactose, and N-acetyllactosamine all produced small but equally intense protein difference spectra with a maximum (Δ? = 2.8 × 10² M^?1 cm^?1) at 291.6 nm. Upon binding of N-dansyl-d-galactosamine to the lectin, there was a fivefold increase in fluorescence intensity of this ligand. The association constant for N-dansyl-d-galactosamine was caused by a very favorable ΔS° of the dansyl group without affecting the strictly carbohydrate-specific character of binding. N-Dansyl-d-galactosamine was employed as a fluorescent indicator ligand in substitution titrations. This involved the use of simple carbohydrates, N-acetyllactosamine, and oligosaccharides which occur in the carbohydrate units of N-glycoproteins; the latter were Gal(β → 4)GlcNAc(β1 → 2)Man, Gal(β1 → 4)GlcNAc(β1 → 6)Man, and Gal(β1 → 4)GlcNAc(β1 → 6)[Gal(β1 → 4)GlcNAc(β1 → 2)]Man. The titrations were performed at two temperatures to determine the thermodynamic parameters. In the series N-acetyl-d-galactosamine, methyl-α-d-galactoside, and lactose, ?ΔH° increased from 24 to 41 kJ mol^?1; it increased further for N-acetyllactosamine and then remained unchanged for the N-acetyllactosamine-containing oligosaccharides (55 ± 1 kJ mol^?1). This indicated that the site specifically accommodated the disaccharide structure with an important contribution of the 2-acetamido group in the penultimate sugar. Beyond this, no additional contacts seemed to be formed. This conclusion also followed from considerations of ΔS° values which became more unfavorable in the above series (?23 to ?101 ± 4 J mol^?1 K^?1); the most negative value of ΔS° was observed with N-acetyllactosamine and the three N-acetyllactosamine-containing oligosaccharides.     

Synthesis and bioassay of isoprenoid 3-alkylthio-1,1,1-trifluoro-2-propanones: Potent,selective inhibitors of juvenile hormone esterase

Four 3-alkylthio-1,1,1-trifluoro-2-propanones with juvenile hormone-like side chains were prepared from citronellol and homogeraniol. These substrates were designed as possible transition-state analogs for the juvenile hormone (JH)-specific esterases present in insects. These four isoprenoid trifluoromethyl ketones were assayed in vitro with JH esterase and general esterases from larvae of the cabbage looper, Trichoplusia ni (Lepidoptera, Noctuidae), and with eel acetylcholinesterase and bovine chymotrypsin. JH esterase inhibition I₅₀ values were in the nanomolar range for all four compounds, while the other esterases had I₅₀'S which were 10³ to 10⁵ higher. The high selectivity of these inhibitors is believed to be due to their similarity in size and functionality to natural JH III. Treatment of T. ni larvae in vivo with solutions of the most active analog, 3-[(E)-4,8-dimethyl-3,7-nonadienylthio]-1,1,1-trifluoro-2-propanone (DNTFP) causes a dose-dependent delay in pupation and a concurrent selective inhibition of JH esterase. These data support the hypothesis that the reduction in in vivo JH titer in larval T. ni is due, in part, to hydrolysis of the hormone by selective esterases. DNTFP appears to be competing with JH for the active site of JH esterase.     

Junction formation in aggregated embryonal carcinoma cells

Under the action of supplemental calcium, H6 mouse embryonal carcinoma cell aggregates undergo compaction, a morphological phenomenon similar to mouse embryonic compaction. Formation of various types of cell junctions, especially gap junctions, is associated with compaction of the embryo and we sought to analyze the pattern of junction formation during aggregation and compaction of H6 cells. At 24 hr of aggregation, gap junctions were abundant in both uncompacted and compacted aggregates but quantitative analysis of freeze fracture replicas of these junctions showed a 20-fold increase in the size of the largest gap junctions in compacted aggregates. Such a difference in size could even be detected at 12 hr of aggregation. Tight junctions were not normally formed in 12 hr aggregates but initial stages of tight junction formation could be noticed in 12 hr compacted aggregates. More definitive tight junctions and desmosomes were evident only after 48 hr of aggregation. Thus we have observed that both uncompacted and compacted aggregates can form gap junctions at similar frequencies, suggesting that cell flattening, which contributes to the compacted morphology, is not a requisite for gap junctions. Likewise, generation of the compacted morphology seems to be independent of gap junction formation. This supports the idea that compaction in embryonal carcinoma cells results from calcium-induced cell flattening, probably through the mobilization of cytoskeletal elements. Calcium-dependent features of H6 cell aggregation and compaction enables the independent analysis of separate steps in compaction.     

Collagen in the egg shell membranes of the hen

Collagen-like proteins have been found in the egg shell membranes of the hen. Materials similar to types I and V collagens were detected in each of the two layers of this membrane, the thick outer membrane and the thin inner membrane. Collagen was extracted by acid-pepsin digestion and isolated by differential salt precipitation. Identification of type-specific collagen-like material was established by coelectrophoresis on SDS-polyacrylamide gels using known collagen standards. These bands were susceptible to digestion by bacterial collagenase. From differential staining of the gels it was estimated that the ratio of collagen types I:V was approximately 100:1. Further confirmation of these biochemical results was obtained with immunofluorescence microscopy using type-specific antisera against chicken types I and V collagen with the indirect sandwich technique. Both the inner and outer shell membranes contained the two types of collagen. Within each membrane, the large, coarse 2.5-micron fibers contained predominantly type I collagen-like material, while type V collagen was mainly associated with the delicate narrower fibers of approximately 0.6-micron diameter. These tended to be concentrated in the inner membrane. At the electron microscopic level, both types of fibers were coated with glycoproteins that stained positively with ruthenium red. The deposition of these collagen-like substances by the hen oviduct on to the surface of the developing egg is an additional example of interstitial-type collagen synthesis and secretion by epithelial rather than by mesenchymal cells.     

Coexistence of fast-type and slow-type C-proteins in neonatal chicken breast muscle

Two different C-protein variants which selectively react with either monoclonal anti-fast C-protein antibody (MF-1) or monoclonal anti-slow C-protein antibody (ALD-66) were separated from neonatal chicken pectoralis muscle by hydroxylapatite column chromatography. Myofibrils isolated from the neonatal chicken muscle reacted with both monoclonal antibodies as examined by an indirect immunofluorescence method. These observations strongly indicate that both fast-type and slow-type C-proteins are expressed in the neonatal chicken skeletal muscle. Both of them are intermingled and assembled in the same myofibrils.     

Microtubule arrays in the cortex and near the germinal vesicle of immature starfish oocytes

An extensive array of long, crisscrossing microtubules has been discovered in the cortex of oocytes of the starfish Pisaster ochraceus. The microtubules were visualized in cortex preparations by indirect immunofluorescence microscopy using antibodies to tubulin. The cortical array of microtubules is present in all oocytes before and for about 30 min after the application of 1-methyladenine, the hormone that induces oocyte maturation. The presence of microtubules was confirmed by electron microscopy. The microtubules in this array are depolymerized when oocytes are treated with colchicine or nocodozole and are augmented when oocytes are treated with taxol. Dihydrocytochalasin B treatment of the oocytes causes the microtubules to aggregate, presumably by altering a microfilament network also found in the cortex. The distribution of microtubules was also explored in whole oocytes stained with antitubulin. One or two aster-like structures were observed adjacent to the germinal vesicle of each oocyte.     

Stage-specific expression of three cell surface carbohydrate antigens during murine spermatogenesis detected with monoclonal antibodies

We have identified three germ cell surface carbohydrate antigens that exhibit a common, stage-specific pattern of expression during spermatogenesis in the mouse. IgM-class monoclonal antibodies designated "J1," "C6," and "A5" were absorbed by adult testis, but not by any adult somatic tissue tested. In indirect immunofluorescence assays using collagenase-dissociated prepuberal and adult testicular cells, these antibodies labeled the surfaces of early and late pachytene spermatocytes and round spermatids. Gonocytes from fetal and neonatal testes were not labeled. In paraffin sections of prepuberal and adult testes, sialidase treatment exposed antigens recognized by antibodies C6 and A5 on preleptotene, leptotene, and zygotene spermatocytes located near the perimeter of seminiferous tubules. The determinants recognized by antibodies J1, C6, and A5 were characterized partially using a sugar hapten inhibition assay. The binding of J1 to adult testicular cells was inhibited specifically by N-acetylglucosamine and the binding of both C6 and A5 was inhibited by N-acetyllactosamine. The glycoconjugates recognized by J1, C6, and A5 eluted from gel filtration columns with an apparent molecular weight greater than 1 X 10(6) and were sensitive to endo-beta-galactosidase (keratanase) treatment. The apparent high molecular weight of these glycoconjugates was confirmed by immunolabeling Western blots of testis extracts separated by SDS-polyacrylamide gel electrophoresis. The results suggest that polylactosamine (keratan) glycoconjugates of high molecular weight are associated with the plasma membranes of meiotic and haploid male germ cells. The effects of sialidase on antibody labeling patterns suggest that changes in cell surface sialylation accompany the transition of early meiotic germ cells to pachytene spermatocytes during spermatogenesis.     

Optimizing conditions for cryopreservation of an insect cell line

A cell line (UM-BGE-2) derived from embryos of the cockroach Blattella germanica was frozen to ?196 °C under a variety of conditions and cell viability was assayed after warming. It was found that cell viability was affected by the cooling rate, the warming rate, the controlled cooling endpoint temperature, and the type and concentration of cryoprotectant. The best survival for cells suspended in Grace's tissue culture medium containing 1 M Me₂SO was obtained when cells were cooled at 1 °C/ min to at least ?90 °C before being placed in liquid nitrogen and warmed at more than 900 °C/min. Cultures initiated from these frozen cells produce typical growth curves and appear normal after several passages.     

Prostaglandin receptors in rat kidney

The physiological effects of prostaglandins (PGs) are mediated through their interactions with specific binding sites (receptors) on effector cells. Since such receptors potentially regulate the action of PGs on the kidney, the distribution and properties of renal PG receptors in the rat were examined. The distribution of PGE2, PGE1, and PGF2 alpha receptors along the nephron was not uniform; the outer medulla had by far the greatest density of sites, followed by the inner medulla and cortex. Receptors were found exclusively in the particulate fractions, of which the 40,000g pellet had the highest specific activity. In the outer medulla, receptor density calculated from Scatchard plots was 2.12 pmol/mg for PGE2, 1.12 for PGE1, and 0.44 for PGF2 alpha; the KD's were similar for all prostaglandins. The conditions for optimal in vitro binding of PGE2 and PGF2 alpha by outer medullary membranes were investigated. In vivo administration of 16,16'-dimethyl-PGE2 resulted in a dose-dependent "down" regulation of PGE2 binding to outer medullary membranes due to changes in both the number and affinities of receptors. Changes in the numbers and/or properties of PG receptors may be an important mechanism for regulating the effects of PGs and renal function under normal and pathologic conditions.