Housekeeping enzymes

The susceptibility of a protein to degradation is expressed as its half-life (t,/2)> the time required to lower its concentration to half the initial value. Half-lives of hver proteins range from under 30 minutes to over 150 hours. Typical housekeeping enzymes have tj/2 values of over 100 hours. By contrast, many key regulatory en-... 

A summary of the steps in the biosynthesis of the porphyrin derivatives from PBG is given in Figure 32-8. The last three enzymes in the pathway and ALA synthase are located in the mitochondrion, whereas the other enzymes are cytosolic. Both erythroid and non-erythroid ( housekeeping ) forms of the first four enzymes are found. Heme biosynthesis occurs in most mammalian cells with the exception of mature erythrocytes, which do not contain mitochondria. However,... 

In mammals there are four genes for the PMCA pump, two of which are expressed in all tissues (housekeeping enzymes), whereas the other two, which have a particularly high affinity for calmodulin, are restricted in their cellular distribution, notably to neurons. Additional variants of PMCAs are produced by alternative splicing of their primary RNA transcripts. 

Most of the antibiotics commercially available nowadays are derivatives of natural compounds produced by bacteria or fungi. It is widely accepted that in nature these secondary metabolites can act as weapons for microbial cell defence, inhibiting the growth of competitors. However, it seems that antibiotics have, in nature, more sophisticated and complex functions [1-3]. Many environmental bacteria can not only cope with natural antimicrobial substances but also benefit from their presence. For instance, the use of antibiotics by bacteria as biochemical signals, modulators of metabolic activity or even carbon sources has been demonstrated [1, 2, 4]. In other cases, antibiotics can be tolerated because they have structures similar to the natural substrates of bacterial housekeeping enzymes and thus are inactivated, leading to a natural form of resistance [2]. These are just some... 

The COX enzyme exists in at least two isoforms. COX-1 is a constitutive or housekeeping isoform that is responsible for the basal production of prostaglandins, prostacyclins, and thromboxanes. COX-2 is inducible by cytokines and other inflammatory stimuli and is believed to predominate during chronic inflammation. The final product of the COX pathway is tissue specific. For example, platelets produce thromboxane A2 (TXA2) vascular endothelial cells produce prostacyclin (PGI2) mast cells produce prostaglandin Dj (PGD2) ... 

Kletzien, R.F., Harris, P.K., FoeUmi, L.A. (1994) Glucose 6-phosphate dehydrogenase a housekeeping enzyme subject to tissue-specific regulation by hormones, nutrients, and oxidant stress. FASEB J. 8, 174-181. 

All plants contain a PEPCase enzyme which, among other likely roles (Vidal et al., 1986), serves to replenish tricarboxylic acid cycle intermediates that are consumed during ammonium assimilation (Latzko Kelly, 1983). The role of this enzyme, other than its presumed housekeeping function, has not been studied in any detail and its activity is probably not controlled by environmental factors. Isoforms of PEPCase have been observed in many plants (C3, C4 and CAM) for example, in rice five isoforms have been detected immunologically and in most plants two to four bands that react with anti-PEPCase antibodies are found (Matsuoka Hata, 1987). It is not clear how these isoforms arise, e.g. by post-translational modification of one form, or whether all of these forms are products of different genes. The housekeeping-type PEPCase enzyme, concerned with anaplerotic functions, is distinct from other PEPCase enzymes that function in plants with C4 and CAM metabolism (see below). 

Two genes encoding different PEPCase enzymes have been identified and characterised in the ice plant (Cushman et al., 1989 Cushman Bohnert, 1989a,b). ppcl is the CAM-specific form, which is expressed predominantly in green tissue, leaves and shoots. A second isogene, ppc2, appears to be responsible for the housekeeping functions of PEPCase and is expressed constitutively at low levels in all tissues of the plant. 

SPCA1 is a housekeeping enzyme and is therefore expressed in all cells (Vanoevelen et al., 2005). Results on the relative expression level in various tissues are still conflicting. Expression levels, both at the mRNA and at the protein level, in rat brain and testis were much higher than in other tissues like lung and liver (Wootton et al., 2004). Such relatively high expression in brain and testis was not observed at the mRNA level in humans (Vanoevelen et al., 2005). The cell-type dependent expression of SPCA1 in various animal species thus deserves further study. 

The cyclooxygenase (COX) enzyme is known to exist as two distinct isoforms. COX-1 is a constitutively expressed housekeeping enzyme found in nearly all tissues and mediates physiological responses. COX-2 is an inducible form expressed primarily by cells involved in the inflammatory response. Several tissues low in CYP expression are rich in COX, which is believed to have significance in the carcinogenic effects of aromatic amines in these organs. 

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