Washington: MIT bio-engineers have found a way to mass-produce smell receptors in the lab, edging closer to creation of 'artificial noses'.

The work could also allow scientists to unlock the mystery of how the sense of smell can recognise an infinite range of odours.

"Smell is perhaps one of the oldest and most primitive senses, but nobody really understands how it works. It still remains a tantalising enigma," said Shuguang Zhang, associate director of MIT's Centre for Biomedical Engineering and co-author of the paper.

Artificial noses could one day replace drug - and explosive - sniffing dogs, and having numerous medical applications, according to Zhang and his colleagues.

Until now, efforts to understand the molecular basis of smell have been stymied by the difficulty in working with the proteins that detect odours, known as olfactory receptors, according to a release of the Massachusetts Institute of Technology (MIT).

"The main barrier to studying smell is that we haven't been able to make enough receptors and purify them to homogeneity. Now, it's finally available as a raw material for people to utilise, and should enable many new studies into smell research," said Brian Cook, who just defended his MIT PhD thesis based on this work.

Smell is one of the most complex and least-understood senses. Humans have a vast olfactory system that includes close to 400 functional genes, more than are dedicated to any other function. Animals such as dogs and mice have around 1,000 functional olfactory receptor genes.

That variety of receptors allows humans and animals to discern tens of thousands of distinct smells. Each odour activates multiple receptors and this pattern of activation creates a signature that the brain can recognise as a particular scent.

The olfactory receptors that bind to odour molecules are membrane proteins, which span the cell surface. Since cell membranes are composed of a double layer of fatty lipid molecules, the receptor proteins are highly hydrophobic (water-fearing).

When such proteins are removed from the cell and placed in water-based solutions, they clump up and lose their structure, said Liselotte Kaiser, co-author of the paper. That makes it very difficult to isolate the proteins in quantities large enough to study them in detail.

Kaiser and others spent several years developing a method to isolate and purify the proteins by performing each step in a hydrophobic detergent solution, which allows the proteins to maintain their structure and function.

The technique involves a cell-free synthesis using commercially available wheat germ extract to produce a particular receptor, then isolating the protein through several purification steps. The method can rapidly produce large amounts of protein - enough to start structural and functional studies.

The work appeared online this week in the Proceedings of the National Academy of Sciences (PNAS). IANS