A stabilised tris(hydroxymethyl)aminomethane adduct in reduced collagen

Tunable bioactivity and mechanics of collagen-based tissue engineering constructs: A comparison of EDC-NHS, genipin and TG2 crosslinkers

Due to its ubiquity and versatility in the human body, collagen is an ideal base material for tissue-engineering constructs. Chemical crosslinking treatments allow precise control of the biochemical and mechanical properties through macromolecular modifications to the structure of collagen. In this work, three key facets regarding the collagen crosslinking process are explored. Firstly, a comparison is drawn between the carbodiimide-succinimide (EDC-NHS) system and two emerging crosslinkers utilising alternate chemistries: genipin and tissue transglutaminase (TG2). By characterising the chemical changes upon treatment, the effect of EDC-NHS, genipin and TG2 crosslinking mechanisms on the chemical structure of collagen, and thus the mechanical properties conferred to the substrate is explored. Secondly, the relative importance of mechanical and biochemical cues on cellular phenomena are investigated, including cell viability, integrin-specific attachment, spreading and proliferation. Here, we observe that for human dermal fibroblasts, long-term, stable proliferation is preconditioned by the availability of suitable binding sites, irrespective of the substrate modulus post-crosslinking. Finally, as seen in the graphical abstract we show that by choosing the appropriate crosslinker chemistries, a materials selection map can be drawn for collagen films, encompassing both a range of tensile modulus and fibroblast proliferation which can be modified independently. Thus, in addition to a range of parameters that can be modified in collagen constructs, we demonstrate a route to obtaining tunable bioactivity and mechanics in collagen constructs is uncovered, that is exclusively driven by the crosslinking process.

Metabolism of diosbulbin B in vitro and in vivo in rats: formation of reactive metabolites and human enzymes involved

Diosbulbin B (DB), a major constituent of the furano-norditerpenes in Dioscorea bulbifera Linn, exhibits potential antineoplasmic activity and hepatotoxicity. The metabolism and reactive metabolites of DB in vitro (with human and animal liver microsomes) and in vivo in rats were investigated. The human enzymes involved in DB metabolism were identified. DB was first catalyzed into reactive metabolites of 2-butene-1,4-dial derivatives dependent on NADPH and then trapped by Tris base or oxidized to hemiacetal lactones (M12 and M13) in microsomal incubations. Tris base was used as buffer constituent and as a trapping agent for aldehyde. Methoxylamine and glutathione (GSH) were also used as trapping agents. DB metabolism in vivo in rats after oral administration was consistent with that in vitro. The structures of M12 and M13, as well as mono-GSH conjugates of DB (M31), were confirmed by nuclear magnetic resonance spectroscopy of the chemically synthesized products. The bioactivation enzymes of DB were identified as CYP3A4/5, 2C9, and 2C19. CYP3A4 was found to be the primary enzyme using human recombinant cytochrome P450 enzymes, specific inhibitory studies, and a relative activity factor approach for pooled human liver microsomes. Michaelis-Menten constants K(m) and V(max) were determined by the formation of M31. The reactive metabolites may be related to the hepatotoxicity of DB. The gender difference in CYP3A expression in mice and rats contributed to the gender-related liver injury and pharmacokinetics in mice and rats, respectively.

The organization of cross-linking in collagen fibrils

The cross-linked peptides in cyanogen bromide digests from rabbit and calf fibrils of type I collagen have been compared by gel filtration and electrophoresis. Fibrils were prepared in vitro from acid-soluble collagen, or tendons were used; both were reduced with borohydride to stabilize cross-linking adducts. The cross-linked peptides were isolated and hydrolyzed, and the reducible cross-links were analyzed. We propose that a prominent pattern of cross-linking involves in-register palisades of molecules overlapping by 27 nm and joined through the condensation of amino-terminal aldol adducts on the carboxy-terminal helical regions of overlapping molecules. Lateral association probably occurs through the carboxy-terminal aldehydes from two molecules forming tri- or quadrivalent adducts with residues in the overlapped molecule. This model favors the recently proposed quasi-hexagonal organization of molecules in fibrils in which rows of molecules occur in lateral register.

Aging, and crosslinking in mammlian collagen

The amount and type of borohydride-reducible crosslinks in collagen have been examined as a function of animal age. In a variety of bovine, canine and human tissues the level of redicible crosslinks decreases with time and the ratios of individual compounds change. There is both tissue and species specificity in the extent of these changes. A decrease in the level of reducible crosslinks correlates with the cessation of growth. Loss of reducible crosslinks does not imply a small total number of crosslinks since physical changes with age imply the opposite. We conclude that reducible crosslinks are converted to a stable nonreducible state and the persistence of low levels of reducible crosslinks may be indicative of a low level of turnover in the tissue. Changes in ratios of reducible crosslinks are of doubtful functioal significance and may simply reflect variation in post-translational modification of lysine residues.