Kir2.1 channels are blocked by Rb+ and Cs+ in a voltage‐dependent manner, characteristic of many inward rectifier K+ channels. Mutation of Ser165 in the transmembrane domain M2 to Leu (S165L) abolished Rb+ blockage and lowered Cs+ blocking affinity. At negative voltages Rb+ carried large inward currents. A model of the Kir2.1 channel, built by homology with the structure of the Streptomyces lividans K+ channel KcsA, suggested the existence of an intersubunit hydrogen bond between Ser165 and Thr141 in the channel pore‐forming P‐region that helps stabilise the structure of this region. However, mutations of Thr141 and Ser165 did not produce effects consistent with a hydrogen bond between these residues being essential for blockage. An alternative alignment between the M2 regions of Kir2.1 and KcsA suggested that Ser165 is itself a pore‐lining residue, more directly affecting blockage. We were able to replace Ser165 with a variety of polar and non‐polar residues, consistent with this residue being pore lining. Some of these changes affected channel blockage. We tested the hypothesis that Asp172 – a residue implicated in channel gating by polyamines – formed an additional selectivity filter by using the triple mutant T141A/S165L/D172N. Large Rb+ and Cs+ currents were measured in this mutant. We propose that both Thr141 and Ser165 are likely to provide binding sites for monovalent blocking cations in wild‐type channels. These residues lie beyond the carbonyl oxygen tunnel thought to form the channel selectivity filter, which the blocking cations must therefore traverse.

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The biogenesis of the mould metabolites rotiorin (11), monascin (111), and rubropunctatin (IV) has been examined by using Me*14C02H and HJ4C02H. In agreement with prediction they arise in part from both sources. The extent of introduction of label from Me*14C0,H into different parts of the molecule does not vary significantly.

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Methods of labelling calciferol in specified positions with tritium have been investigated with the purpose of selecting a stable and highly radioactive compound for metabolic studies. [3 α - 3 H]Ergocalciferol and [24, 25, 26, 27- 3 H 9 ]-, [7- 3 H]-, [6- 3 H]- and [1 α - 3 H]-cholecalciferol have been prepared and the excretion and distribution in the body of vitamin D activity and radioactivity after administration to young rachitic rats has been studied. It is concluded that [1 α - 3 H]cholecalciferol is a suitable tritium-labelled form of vitamin D for further meta­bolic studies. The more notable results of the present studies are that much of the tritium from variously tritiated calciferols finds its way into the body water, and that break-down products of calciferol are to be found in the more polar fractions of the extracts of faeces, into which they pass from the bile.