Measurement of anisotropic mechanical properties in porcine brain white matter ex vivo using magnetic resonance elastography.

The mechanical properties of brain tissue, particularly those of white matter (WM), need to be characterized accurately for use in finite element (FE) models of brain biomechanics and traumatic brain injury (TBI). Magnetic resonance elastography (MRE) is a powerful tool for non-invasive estimation of the mechanical properties of soft tissues. While several studies involving direct mechanical tests of brain tissue have shown mechanical anisotropy, most MRE studies of brain tissue assume an isotropic model. In this study, an incompressible transversely isotropic (TI) material model parameterized by minimum shear modulus (µ2), shear anisotropy parameter (ϕ), and tensile anisotropy parameter (ζ) is applied to analyze MRE measurements of ex vivo porcine white matter (WM) brain tissue. To characterize shear anisotropy, "slow" (pure transverse) shear waves were propagated at 100, 200 and 300Hz through sections of ex vivo brain tissue including both WM and gray matter (GM). Shear waves were found to propagate with elliptical fronts, consistent with TI material behavior. Shear wave fields were also analyzed within regions of interest (ROI) to find local shear wavelengths parallel and perpendicular to fiber orientation. FE simulations of a TI material with a range of plausible shear modulus (µ2) and shear anisotropy parameters (ϕ) were run and the results were analyzed in the same fashion as the experimental case. Parameters of the FE simulations which most closely matched each experiment were taken to represent the mechanical properties of that particular sample. Using this approach, WM in the ex vivo porcine brain was found to be mildly anisotropic in shear with estimates of minimum shear modulus (actuation frequencies listed in parenthesis): µ2= 1.04 ± 0.12 kPa (at 100Hz), µ2= 1.94 ± 0.29 kPa (at 200Hz), and µ2= 2.88 ± 0.34 kPa (at 300Hz) and corresponding shear anisotropy factors of ϕ= 0.27 ± 0.09 (at 100Hz), ϕ= 0.29 ± 0.14 (at 200Hz) and ϕ= 0.34 ± 0.13 (at 300Hz). Future MRE studies will focus on tensile anisotropy, which will require both slow and fast shear waves for accurate estimation.

Conversion of canavanine to alpha-keto-gamma-guanidinooxybutyrate and to vinylglyoxylate and 2-hydroxyguanidine.

It was observed previously that hydroxyguanidine is formed in the reaction of canavanine(2-amino-4-guanidinooxybutanoate) with amino acid oxidases. The present work shows that hydroxyguanidine is formed by a nonenzymatic beta,gamma-elimination reaction following enzymatic oxidation at the alpha-C and that the abstraction of the beta-H is general-base catalyzed. The elimination reaction requires the presence in the alpha-position of an anion-stabilizing group--the protonated imino group (iminium ion group) or the carbonyl group. The iminium ion group is more activating than the carbonyl group. Elimination is further facilitated by protonation of the guanidinooxy group. The other product formed in the elimination reaction was identified as vinylglyoxylate (2-oxo-3-butenoate), a very highly electrophilic substance. The product resulting from hydrolysis following oxidation was identified as alpha-keto-gamma-guanidinooxybutyrate (ketocanavanine). The ratio of hydroxyguanidine to ketocanavanine depended upon the concentration and degree of basicity of the basic catalyst and on pH. In the presence of semicarbazide, the elimination reaction was prevented because the imino group in the semicarbazone derivative of ketocanavanine is not significantly protonated. Incubation of canavanine with 5'-deoxypyridoxal also yielded hydroxyguanidine. Since the elimination reactions take place under mild conditions, they may occur in vivo following oxidation at the alpha-C of L-canavanine (ingested or formed endogenously) or of other amino acids with a good leaving group in the gamma-position (e.g., S-adenosylmethionine, methionine sulfoximine, homocyst(e)ine, or cysteine-homocysteine mixed disulfide) by an L-amino acid oxidase, a transaminase, or a dehydrogenase. Therefore, vinylglyoxylate may be a normal metabolite in mammals which at elevated concentrations may contribute to the in vivo toxicity of canavanine and of some of the other above-mentioned amino acids.