| beta-lactam resistance | Nonsusceptibility of an organism to the action of the beta-lactam antibiotics. (12 Dec 1998) |
|---|---|
| beta limit | <radiobiology> if the plasma pressure in a tokamak becomes too high, the so-called ballooning modes become unstable and lead to a loss of confinement (sometimes catastrophic, sometimes not). The exact value at which this occurs depends strongly on the magnetic field B, the plasma minor radius a, and the toroidal plasma current I, such that maximum value of the normalised beta, beta_N=betaBa/I, is around 4% (with B in Teslas, a in metres, and I in Mega-amperes). The exact value depends on details of the plasma shape, the plasma profiles, and the safety factor. Synonym: troyon limit. (09 Oct 1997) |
| beta-mercaptoethanol | HO-CH2-CH2-SH;a reagent used to reduce disulfide bonds, particularly in proteins, and to prevent their formation. Synonym: beta-mercaptoethanol. (05 Mar 2000) |
| beta-n-acetylhexosaminidase | <enzyme> An enzyme hydrolyzing terminal non-reducing n-acetyl-d-hexosamine residues in n-acetyl-beta-d-hexosaminides. It acts on glucosides, galactosides, and several oligosaccharides. Hexosaminidase a cleaves gm2, ga2, globosides, and hexosamine oligosaccharides. Deficiency of this isoenzyme causes tay-sachs disease. Hexosaminidase b cleaves all the above substrates except gm2. A deficiency of both a and b causes sandhoff disease. The enzyme has also been used as a tumour marker to distinguish between malignant and benign disease. Chemical name: beta-N-Acetyl-D-hexosaminide N-acetylhexosaminohydrolase Registry number: EC 3.2.1.52 (12 Dec 1998) |
| beta-naphthoflavone | <chemical> A polyaromatic hydrocarbon inducer of p4501a1 and p4501a2 cytochromes. (proc soc exp biol med 1994 dec:207(3):302-308) Pharmacological action: enzyme inhibitors. (12 Dec 1998) |
| beta-normal | <radiobiology> Beta-N, the normalised beta, is beta relative to the beta limit. (09 Oct 1997) |
| beta-oxidation | <biochemistry> The oxidative breakdown of fatty acids into acetyl-coenzyme A by repeated oxidation at the beta-carbon atom. Oxidation of the beta-carbon (carbon 3) of a fatty acid causes the formation of the beta-keto (beta-oxo) acid analog. This is of importance in fatty acid catabolism, the entire pathway for the catabolism of saturated fatty acids containing an even number of carbon atoms. Beta-oxidation is a part of this pathway and acetyl-CoA is a major product of this pathway. (05 Mar 2000) |
| beta-oxidation-condensation theory | <biochemistry> That the two carbon fragments split from the fatty acid molecule by beta-oxidation are converted to acetic acid and then condensed to acetoacetic acid. (05 Mar 2000) |
| beta particle | <physics, radiobiology> An electron or positron emitted from a radioactive nucleus during beta decay. (09 Oct 1997) |
| beta pleated sheet | <cell biology> Beta secondary structure in proteins consists of two almost fully extended polypeptide chains lying side by side, linked by interchain hydrogen bonds between peptide C=O and N H groups. The chains may run in the same or opposite directions (yielding parallel or antiparallel structures, respectively). When multiple chains are involved, an extended sheet, the pleated sheet, is formed. (18 Nov 1997) |
| beta-poloidal | <radiobiology> Beta-P is the same as the ordinary beta, except only the poloidal field is used in calculating the magnetic field pressure. Beta-P is greater than 1 in many modern tokamaks. (09 Oct 1997) |
| beta radiation | <radiobiology> Radiant energy from a source of beta rays. (05 Mar 2000) |
| beta ray | 1. <radiobiology> Original term used for electrons (and positrons) ejected from decaying nuclei via beta emission. (Label derives from the old days when we had various kinds of radiation emission, and they were labelled alpha, beta, and gamma (the first letters of the Greek Alphabet) because no one really knew what any of them were.) 2. A stream of positive or negative electrons ejected with high energy from a disintegrating atomic nucleus; most biomedically used isotopes emit negative particles (electrons or negatrons, rather than positrons). Cathode rays are low-energy negative electrons produced in cathode ray tubes, also called television tubes or oscilloscopes. (12 Sep 2000) |
| beta rhythm | <neurology> Brain waves in the electroencephalogram which have a frequency of 18 to 30 per second. They are typical during periods of intense activity of the nervous system, and occur principally in the parietal and frontal regions. (12 Dec 1998) |
| beta sheets | <biochemistry> A structure of proteins where the peptide is extended and stabilised by hydrogen bonding between NH and CO groups of different polypeptide chains or separate regions of the same chain. (05 Mar 2000) |