Characterization of casein and alpha lactalbumin of African elephant (Loxodonta africana) milk.

The current research reports partial characterization of the caseins and α-lactalbumin (α-LA) of the African elephant with proposed unique structure-function properties. Extensive research has been carried out to understand the structure of the casein micelles. Crystallographic structure elucidation of caseins and casein micelles is not possible. Consequently, several models have been developed in an effort to describe the casein micelle, specifically of cow milk. Here we report the characterization of African elephant milk caseins. The κ-caseins and ß-caseins were investigated, and their relative ratio was found to be approximately 1:8.5, whereas α-caseins were not detected. The gene sequence of ß-casein in the NCBI database was revisited, and a different sequence in the N-terminal region is proposed. Amino acid sequence alignment and hydropathy plots showed that the κ-casein of African elephant milk is similar to that of other mammals, whereas the ß-casein is similar to the human protein, and displayed a section of unique AA composition and additional hydrophilic regions compared with bovine caseins. Elephant milk is destabilized by 62% alcohol, and it is speculated that the ß-casein characteristics may allow maintenance of the colloidal nature of the casein micelle, a role that was previously only associated with κ-casein. The oligosaccharide content of milk was reported to be low in dairy animals but high in some other species such as humans and elephants. In the milk of the African elephant, lactose and oligosaccharides both occur at high levels. These levels are typically related to the content of α-LA in the mammary gland and thus point to a specialized carbohydrate synthesis, where the whey protein α-LA plays a role. We report the characterization of African elephant α-LA. Homology modeling of the α-LA showed that it is structurally similar to crystal structures of other mammalian species, which in turn may be an indication that its functional properties, such as lactose synthesis, should not be impaired.

Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?

Previous studies of the subsurface biosphere have deduced average cellular doubling times of hundreds to thousands of years based upon geochemical models. We have directly constrained the in situ average cellular protein turnover or doubling times for metabolically active micro-organisms based on cellular amino acid abundances, D/L values of cellular aspartic acid, and the in vivo aspartic acid racemization rate. Application of this method to planktonic microbial communities collected from deep fractures in South Africa yielded maximum cellular amino acid turnover times of ~89 years for 1 km depth and 27 °C and 1-2 years for 3 km depth and 54 °C. The latter turnover times are much shorter than previously estimated cellular turnover times based upon geochemical arguments. The aspartic acid racemization rate at higher temperatures yields cellular protein doubling times that are consistent with the survival times of hyperthermophilic strains and predicts that at temperatures of 85 °C, cells must replace proteins every couple of days to maintain enzymatic activity. Such a high maintenance requirement may be the principal limit on the abundance of living micro-organisms in the deep, hot subsurface biosphere, as well as a potential limit on their activity. The measurement of the D/L of aspartic acid in biological samples is a potentially powerful tool for deep, fractured continental and oceanic crustal settings where geochemical models of carbon turnover times are poorly constrained. Experimental observations on the racemization rates of aspartic acid in living thermophiles and hyperthermophiles could test this hypothesis. The development of corrections for cell wall peptides and spores will be required, however, to improve the accuracy of these estimates for environmental samples.

Aerobic Cr(VI) reduction by Thermus scotoductus strain SA-01.

AIM: To evaluate Thermus scotoductus SA-01's ability to reduce Cr(VI) aerobically. METHODS AND RESULTS: T. scotoductus SA-01 is able to reduce Cr(VI) aerobically when grown in a complex organic medium containing Cr(VI) concentrations up to 0.5 mmol l(-1). Suspension of T. scotoductus SA-01 cells also reduced Cr(VI) aerobically under nongrowth conditions using a variety of electron donors as well as in the absence of an exogenous electron donor. The optimum temperature and pH for Cr(VI) reduction under nongrowth conditions were found to be 80 degrees C and 7, respectively. It was also found that the Cr(VI) reduction was catalysed by a cytoplasmic, constitutively expressed enzyme. CONCLUSIONS: Apart from SA-01's ability to reduce Cr(VI) through a strictly anaerobic membrane-bound mechanism (unpublished data), it also has a second enzyme localized in the cytoplasm that can reduce Cr(VI) aerobically. As this enzyme is constitutively expressed and not induced by Cr(VI), it remains to be determined whether it has any other physiological functions. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report of a Thermus species able to reduce Cr(VI) aerobically and extends the knowledge of parameters associated with Cr(VI) reduction. Employing thermophiles in bioremediation using industrial bioreactors would cancel the need for expensive cooling systems.