Activity assays cell components

Plants are not different from other natural product samples in that they too tend to interfere with various screening formats in nonspecific ways as nuisance compounds displaying unwanted color, inherent fluorescence, promiscuous or aggregate behaviors, detergent-like activities, or toxicity (Feng et al., 2005 Appleton, Buss, and Butler, 2007). Biochemical assays (cell-free defined systems) are notoriously sensitive to such interference by natural product extracts. Cell-based reporter assays and cell-based so-called phenotypic screens always require parental cell controls to determine extract toxicity. However, plants contain their own sets of components that are problematic to screening assays and compound identification. 

The design of assay systems is another particularly important factor for testing the sample compounds. Assays have to be specific and sensitive. The assays used for HTS come in many forms. There are binding assays, or enzyme-based or cell-based assays. Cell-based assays have become an important test compared with other in vitro assays, as they can provide information about bioavailability, cytotoxicity and effects on biochemical pathway. Invariably, the enzyme-based and cell-based assay systems consist of receptors or mimetics of receptors (components that mimic active parts of receptors). Normally the assays are linked to an indicator that shows the ligand-receptor interaction as some forms of signal. Radioligand binding... 

The erythrocyte transketolase (ETK) activation assay (also known as the saturation test) measures the functional capacity of the enzyme transketolase in red blood cells i.e. erythrocytes). Transketolase is a thiamine-dependent enzyme in the non-oxidative branch of the pentose phosphate pathway (PPP), a process of glucose turnover that produces nicotinamide adenine dinucleotide phosphates (NADPH) as reducing equivalents and pentose sugars as essential components of nucleotides. In the absence of adequate thiamine, the PPP output is compromised. 

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