ENTRIES A–Z
Philip Winn in Dictionary of Biological Psychology, 2003
Digestion is one of the four stages through which food is processed: these are ingestion digestion, absorption and elimination. Ingestion is the act of FEEDING; digestion is the process of degrading food components into their MOLECULE constituents for use— molecules small enough to be transported across a cell MEMBRANE have to be generated. CARBOHYDRATES are broken down to simple SUGARS (principally GLUCOSE), FATS to GLYCEROL and FATTY ACIDS, PROTEINS to AMINO ACIDS) and NUCLEIC ACIDS to NUCLEOTIDES. The essential process in this is known as enzymatic hydrolysis—the addition of water to molecules by ENZYME activity, causing them to break down. The third stage, absorption (the absorptive phase) involves cells taking up the molecules presented by digestion; the final stage is elimination, the voiding of waste material.
Technetium-Labeled Compounds
Garimella V. S. Rayudu, Lelio G. Colombetti in Radiotracers for Medical Applications, 2019
Oxidation of tin(II) occurs according to the reaction Sn2+ ⇌ Sn4+ + 2e−, namely by the removal of two electrons by an oxidizing agent. Hydrolysis involves many complicated reactions resulting in either a precipitate of the oxides or the formation of soluble hydroxides or basic salts. Even though these two reactions — hydrolysis and oxidation — involve separate and different mechanisms, often in practice they take place simultaneously and are extremely difficult to control. The predominance of one over the other cannot be ascertained either. As a result, the use of stannous tin complexed with suitable ligands (which stabilize it to different degrees towards oxidation and hydrolysis) is often resorted to. In cases where ionic Sn(II) is required, careful exclusion of oxygen and other oxidizing materials and impurities is mandatory. Fortunately, in radiopharmaceutical use, one can often substitute stannous chloride with compounds like stannous fluoride, stannous tartrate, stannous citrate, etc. in many situations without loss of effectiveness. For example, when animals are given stannous chloride in their drinking water, almost all the Sn(II) is lost within 6 to 10 hr whereas only 20 to 30% loss is experienced over 24 hr using stannous fluoride because of the formation of fluoride complexes which confer hydrolytic stability on the Sn(II) ion. Several bone agent kits involve a similar loss in their Sn(II) content over a period of time. Use of ascorbate and other “stabilizers” helps prevent this loss to an appreciable extent thus prolonging the useful life of the kits before as well as after reconstitution with pertechnetate.125, 126
Proteins in Cosmetics
E. Desmond Goddard, James V. Gruber in Principles of Polymer Science and Technology in Cosmetics and Personal Care, 1999
Fishes are a profitable source of native collagen since, in contrast to mammalian collagens, the extractability of the fish skin collagen in acid is higher and does not change significantly with age in many species. Soluble collagen obtained from fish is cheaper than the bovine-originated product; it has, however, lower stability to thermal denaturation (see Section III.A.6). Hydrolysis
Fluid flow effects on the degradation kinetics of bioresorbable polymers
Published in Computer Methods in Biomechanics and Biomedical Engineering, 2021
Zhitao Liu, Hongbo Zhang, Huanxin Lai
Hydrolysis is known to be the dominant mechanism in hydrolytic degradation. The diffusion and accumulation of water molecules causes hydrolysis of the ester bonds in the polymer matrix. As a result, these chains are split into water soluble shorter chains (oligomers and monomers) characterized by carboxylic ends. Because of the chain scission, the molecular weight of the polymer decreases gradually. The degradation products then diffuse into the surrounding environment, which results in the mass loss of polymers. The diffusion is generally slower in a large-size device, due to the greater diffusion distance (Siepmann et al. 2005; Xu et al. 2017). The slow diffusion may result in accumulation of acidic products inside the polymer matrix, and they accelerate the degradation process. The phenomenon is known as the autocatalysis (Gentile et al. 2014; Laycock et al. 2017).
New approaches to tumor therapy with siRNA-decorated and chitosan-modified PLGA nanoparticles
Published in Drug Development and Industrial Pharmacy, 2019
The two main release mechanisms associated with drug release from PLGA NP systems are diffusion and degradation/erosion. The drug release rate is initially controlled by diffusion followed by degradation/erosion [48,49]. There seems to be an inverse relationship between the amount of drug released and particle size. Large microspheres degrade faster than small microspheres [48,50]. This is probably due to the increased accumulation of acidic products during polymer hydrolysis in large microspheres where hydrolysis starts immediately in PLGA systems. Further catalysis of hydrolysis by the acids produced during initial hydrolysis, i.e. the autocatalytic process, leads to more rapid degradation at the center than at the surface of the PLGA matrix. This effect becomes more pronounced as the size of the NP system increases [46]. The F1 formulation with a larger particle size probably degraded relatively faster than F2, F3, and F4, leading to higher release. This hypothesis is further supported by the fact that the F2 formulation had the smallest particle size and the lowest release rate. In addition, slow release rates may be due to both the natural structure of PLGA and the modification of NPs with chitosan [51]. Figure 1(a,b) shows rapid release up to 24 h. Cumulative release rates with burst release at the 24th hour for formulations F1, F2, F3, and F4 were 29.3 ± 2.4%, 26.0 ± 2.8%, 37.5 ± 1.6%, and 30.2 ± 2.6%, respectively. The rapid release of these formulations stopped at the end of the 24th hour and passed to the second phase-slow release. Figure 1(a) clearly shows biphasic release in the NP formulations [52].
Development of stabilized tenofovir disoproxil tablet: degradation profile, stabilization, and bioequivalence in beagle dogs
Published in Drug Development and Industrial Pharmacy, 2018
Ga-Hui Oh, Joo-Eun Kim, Young-Joon Park
Hydrolysis is the most common process among factors associated with degradation such as pH and oxidation conditions. Hydrolysis rates depend on exposure to acidic or basic conditions and the drug concentrations [17,18]. The susceptibility of the test drug substance to hydrolysis and oxidation was evaluated in 0.01 N HCl, 0.01 N NaOH, and 3% H2O2 solutions. After 24 h, the TD tablet was more stable in the acidic solution than it was in the alkali and H2O2 solutions (Figure 2(B)). The higher instability of TD under basic conditions is attributable to the presence of P–O and C–O bonds, which are specifically resistant to attacks by the nucleophiles in alkaline solution. In addition, the oxidative degradation of TD involves an electron transfer mechanism that forms reactive anions and cations. Especially, amines, sulfides, and phenols are liable to electron transfer oxidation [18]. Therefore, the generation of dimers by oxidation may be due to the presence of amines in the tenofovir structure. The results of the degradation profiles indicate that TD is readily converted to tenofovir and tenofovir monoester in stress conditions, resulting in a similar response as that of TDF to oxidative, thermal, and hydrolysis conditions [19].
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