Research from the US could lead to improved high throughput screening for drug discovery.

Many small organic molecules can form aggregates in solution. Typically around 200nm in size, the aggregates can inhibit enzymes in vitro. The protein bovine serum albumin (BSA) can be used to minimise this inhibition but, until now, little was known about the mechanism.

Brian Shoichet and Kristin Coan from the University of California, San Francisco, tested the effects of protein solutions on enzyme inhibition by aggregates. They found that BSA is not unique in minimising enzyme inhibition since other proteins have a similar effect. They also found that, in the absence of protein, the aggregates formed a tight, non-reversible complex with the enzyme, which could not be disrupted by subsequently adding protein. This suggests that the proteins prevent inhibition by saturating the aggregates before they bind to the enzyme, rather than by disrupting the aggregates afterwards.

According to Shoichet, one of the biggest problems in high throughput screening for new leads in drug discovery is false positive hits. Aggregates are key players in causing these as they inhibit enzymes non-specifically, he explained. Timothy Dore, an expert in small molecule probes for biological systems from the University of Georgia in Athens, US, said Shoichet and Coan's results suggest that using large protein concentrations might be one way of minimising the number of these false positives.

Shoichet and Coan's protein solutions were prepared at concentrations that mimic physiological conditions. 'This work also supports the idea that aggregates exist in biological environments, such as cells, tissues, or whole animals, where protein concentrations are high,' said Dore. 

Shoichet said that although the physiological behaviour of aggregates is poorly understood, it is possible that drug aggregates also exist in these conditions. This would greatly influence how we study drug distribution and pharmacology, he said.

Katherine Vickers