Search

Certain amino acids and other compounds are metabolized via: Propionyl-CoA carboxylase in equilibrium D-methylmalonyl-CoA racemase in equilibrium L-methylmalonyl-CoA (adenosylcobalamin) mutase in equilibrium succinyl-CoA in equilibrium tricarboxylic acid cycle. In cobalamin deficiency and in genetic disorders involving adenosylcobalamin or the mutase, large amounts of methylmalonic acid are excreted in the urine. Its origin is unknown, however, since nonesterified methylmalonic acid is not present in the above or other known pathways. To investigate the origin of methylmalonic acid, we fractionated rat liver by gel filtration and found a single peak (Mr = 35,000) of activity for the hydrolysis of DL-methylmalonyl-CoA to methylmalonic acid and CoA. The enzyme has been purified 3,100-fold with a yield of 2.1% from 1.6 kg of rat liver using a purification scheme consisting of ammonium sulfate fractionation, ion exchange chromatography on CM (carboxymethyl)-cellulose and DEAE-cellulose, affinity chromatography on CoA-agarose, and gel filtration on Sephadex G-100. The final preparation gave a single band on polyacrylamide gel electrophoresis. The molecular weight of the purified enzyme is 35,000 based on gel filtration, and sodium dodecyl sulfate-polyacrylamide electrophoresis in the presence and absence of 1% 2-mercaptoethanol. The enzyme was shown to be active on the D-isomer, but not on the L-isomer, of methylmalonyl-CoA by CD spectropolarimetry. The Km for D-methylmalonyl-CoA is 0.7 mM and the molar activity is 1,400 molecules of substrate/min/molecule of enzyme. At substrate concentrations of 0.5 mM, the relative rate of hydrolysis of CoA esters was as follows: D-methylmalonyl-CoA (100%), malonyl-CoA (16%), propionyl-CoA (3%), acetyl-CoA (1%), succinyl-CoA (less than 1%), and palmitoyl-CoA (less than 1%). This enzyme appears to account for the markedly increased amounts of methylmalonic acid that are excreted in the urine in cobalamin deficiency and in genetic disorders involving adenosylcobalamin or L-methylmalonyl-CoA mutase.

A laser milling process employing a pulsed, carbon dioxide laser has been investigated using graphite as a model material. Material is removed by scanning the focused beam across the surface of the workpiece leaving behind a series of narrow, parallel, overlapping grooves. These grooves, together, constitute the removal of a thin layer of uniform thickness lying parallel to a layer plane. In order to remove layers bounded at the edge by upright walls perpendicular to the layer plane, the laser beam must be tilted with respect to the layer plane. Using this approach, it is possible to produce perpendicular steps and cylindrical surfaces.

In this research, the feasibility of shaping Si3N4 by overlapping multiple grooves produced with a continuously operated CO2 laser beam is demonstrated. The relationships of process parameters such as material removal rate and arithmetic average surface roughness to machine parameters such as feed and speed have been investigated. Strategies for laser shaping are discussed and an economic evaluation of laser shaping is presented.

The periodic morphology of cube shaped, coherent γ ′ [Ni3(Al, Ti)] precipitates in a representative nickel-base superalloy single crystal is shown to be influenced by the application of a 〈100〉 oriented uniaxial stress during annealing at elevated temperature. Furthermore, the stress annealed γ′ morphologies depend on the stress sense. Tensile and compressive stress annealing result in a directional coarsening of precipitates, and then agglomeration to form γ ′ precipitate plates with broad faces aligned perpendicular to the stress axis and precipitate parallelepipeds with long axes parallel to the stress axis, respectively. In explaining the morphological changes that occur during stress annealing, we consider two driving forces: i) a driving force due to changes in the stress-free shape of the specimen, and ii) a driving force due to changes in the effective modulus of the specimen. The first, which depends on a difference in the elastic constants and the lattice parameters of the two phases, is apparently the dominant driving force for the observed morphological changes in a stress annealed superalloy. The second, which depends only on a difference in elastic constants, may be important in alloys with elastically soft, incoherent inclusions such as pores.

Abstract: The effect of thermally induced residual stresses on the yield behavior of a unidirectionally solidified eutectic, (Co, Cr)-(Cr, Co)7C3 is presented. At low temperatures, the yield stress is found to depend on the sense of the applied stress. The difference in yield stress between tension and compression is a function of temperature and disappears at a sufficiently high stress relaxation temperature. A straightforward analysis is presented that predicts the observed yielding behavior and a stress relaxation temperature that agrees well not only with the value obtained by observing the temperature dependence of the yield stress, but also with the value obtained from the thermal expansion behavior of the eutectic composite.